From 14911c6f48ec9571343ac36ae02f2db68bf9e7f9 Mon Sep 17 00:00:00 2001 From: Wolfram Sang Date: Tue, 28 Nov 2017 16:53:32 +0100 Subject: i2c: gpio: add fault injector Add fault injection capabilities to the i2c-gpio driver. When connected to another I2C bus, it can create unusual states which the other I2C bus master driver needs to handle. Only for debugging! Signed-off-by: Wolfram Sang --- Documentation/i2c/gpio-fault-injection | 54 ++++++++++++++++++++++++++++++++++ 1 file changed, 54 insertions(+) create mode 100644 Documentation/i2c/gpio-fault-injection (limited to 'Documentation/i2c') diff --git a/Documentation/i2c/gpio-fault-injection b/Documentation/i2c/gpio-fault-injection new file mode 100644 index 0000000..e0c4f77 --- /dev/null +++ b/Documentation/i2c/gpio-fault-injection @@ -0,0 +1,54 @@ +Linux I2C fault injection +========================= + +The GPIO based I2C bus master driver can be configured to provide fault +injection capabilities. It is then meant to be connected to another I2C bus +which is driven by the I2C bus master driver under test. The GPIO fault +injection driver can create special states on the bus which the other I2C bus +master driver should handle gracefully. + +Once the Kconfig option I2C_GPIO_FAULT_INJECTOR is enabled, there will be an +'i2c-fault-injector' subdirectory in the Kernel debugfs filesystem, usually +mounted at /sys/kernel/debug. There will be a separate subdirectory per GPIO +driven I2C bus. Each subdirectory will contain files to trigger the fault +injection. They will be described now along with their intended use-cases. + +"scl" +----- + +By reading this file, you get the current state of SCL. By writing, you can +change its state to either force it low or to release it again. So, by using +"echo 0 > scl" you force SCL low and thus, no communication will be possible +because the bus master under test will not be able to clock. It should detect +the condition of SCL being unresponsive and report an error to the upper +layers. + +"sda" +----- + +By reading this file, you get the current state of SDA. By writing, you can +change its state to either force it low or to release it again. So, by using +"echo 0 > sda" you force SDA low and thus, data cannot be transmitted. The bus +master under test should detect this condition and trigger a bus recovery (see +I2C specification version 4, section 3.1.16) using the helpers of the Linux I2C +core (see 'struct bus_recovery_info'). However, the bus recovery will not +succeed because SDA is still pinned low until you manually release it again +with "echo 1 > sda". A test with an automatic release can be done with the +'incomplete_transfer' file. + +"incomplete_transfer" +--------------------- + +This file is write only and you need to write the address of an existing I2C +client device to it. Then, a transfer to this device will be started, but it +will stop at the ACK phase after the address of the client has been +transmitted. Because the device will ACK its presence, this results in SDA +being pulled low by the device while SCL is high. So, similar to the "sda" file +above, the bus master under test should detect this condition and try a bus +recovery. This time, however, it should succeed and the device should release +SDA after toggling SCL. Please note: there are I2C client devices which detect +a stuck SDA on their side and release it on their own after a few milliseconds. +Also, there are external devices deglitching and monitoring the I2C bus. They +can also detect a stuck SDA and will init a bus recovery on their own. If you +want to implement bus recovery in a bus master driver, make sure you checked +your hardware setup carefully before. -- cgit v1.1 From d4e01186ae1c6045b5a508741f2446dffec7511c Mon Sep 17 00:00:00 2001 From: Wolfram Sang Date: Sat, 4 Nov 2017 21:20:07 +0100 Subject: i2c: add docs to clarify DMA handling Reviewed-by: Jonathan Cameron Reviewed-by: Mauro Carvalho Chehab Signed-off-by: Wolfram Sang Signed-off-by: Wolfram Sang --- Documentation/i2c/DMA-considerations | 67 ++++++++++++++++++++++++++++++++++++ 1 file changed, 67 insertions(+) create mode 100644 Documentation/i2c/DMA-considerations (limited to 'Documentation/i2c') diff --git a/Documentation/i2c/DMA-considerations b/Documentation/i2c/DMA-considerations new file mode 100644 index 0000000..966610a --- /dev/null +++ b/Documentation/i2c/DMA-considerations @@ -0,0 +1,67 @@ +================= +Linux I2C and DMA +================= + +Given that i2c is a low-speed bus, over which the majority of messages +transferred are small, it is not considered a prime user of DMA access. At this +time of writing, only 10% of I2C bus master drivers have DMA support +implemented. And the vast majority of transactions are so small that setting up +DMA for it will likely add more overhead than a plain PIO transfer. + +Therefore, it is *not* mandatory that the buffer of an I2C message is DMA safe. +It does not seem reasonable to apply additional burdens when the feature is so +rarely used. However, it is recommended to use a DMA-safe buffer if your +message size is likely applicable for DMA. Most drivers have this threshold +around 8 bytes (as of today, this is mostly an educated guess, however). For +any message of 16 byte or larger, it is probably a really good idea. Please +note that other subsystems you use might add requirements. E.g., if your +I2C bus master driver is using USB as a bridge, then you need to have DMA +safe buffers always, because USB requires it. + +Clients +------- + +For clients, if you use a DMA safe buffer in i2c_msg, set the I2C_M_DMA_SAFE +flag with it. Then, the I2C core and drivers know they can safely operate DMA +on it. Note that using this flag is optional. I2C host drivers which are not +updated to use this flag will work like before. And like before, they risk +using an unsafe DMA buffer. To improve this situation, using I2C_M_DMA_SAFE in +more and more clients and host drivers is the planned way forward. Note also +that setting this flag makes only sense in kernel space. User space data is +copied into kernel space anyhow. The I2C core makes sure the destination +buffers in kernel space are always DMA capable. Also, when the core emulates +SMBus transactions via I2C, the buffers for block transfers are DMA safe. Users +of i2c_master_send() and i2c_master_recv() functions can now use DMA safe +variants (i2c_master_send_dmasafe() and i2c_master_recv_dmasafe()) once they +know their buffers are DMA safe. Users of i2c_transfer() must set the +I2C_M_DMA_SAFE flag manually. + +Masters +------- + +Bus master drivers wishing to implement safe DMA can use helper functions from +the I2C core. One gives you a DMA-safe buffer for a given i2c_msg as long as a +certain threshold is met:: + + dma_buf = i2c_get_dma_safe_msg_buf(msg, threshold_in_byte); + +If a buffer is returned, it is either msg->buf for the I2C_M_DMA_SAFE case or a +bounce buffer. But you don't need to care about that detail, just use the +returned buffer. If NULL is returned, the threshold was not met or a bounce +buffer could not be allocated. Fall back to PIO in that case. + +In any case, a buffer obtained from above needs to be released. It ensures data +is copied back to the message and a potentially used bounce buffer is freed:: + + i2c_release_dma_safe_msg_buf(msg, dma_buf); + +The bounce buffer handling from the core is generic and simple. It will always +allocate a new bounce buffer. If you want a more sophisticated handling (e.g. +reusing pre-allocated buffers), you are free to implement your own. + +Please also check the in-kernel documentation for details. The i2c-sh_mobile +driver can be used as a reference example how to use the above helpers. + +Final note: If you plan to use DMA with I2C (or with anything else, actually) +make sure you have CONFIG_DMA_API_DEBUG enabled during development. It can help +you find various issues which can be complex to debug otherwise. -- cgit v1.1