i2c-dev mismatch with proprietary nvidia driver

[Sanford Rockowitz <rockowitz@xxxxxxxxxxx>]

I'm not sure where to post this bug report on the Linux side, I have 
already posted it at developer.nvida.com.  If there's a more appropriate 
location, please let me know.

At approximately line 41 of nvidia driver file nv-i2c.c, in function 
nv_i2c_algo_master_transfer(), there's an if-test for symbol 
I2C_M_DMA_SAFE.  If the symbol is not defined, a call to the function 
with the bit set fails with error -EINVAL. The file includes 
"linux/i2c.h", so apparently there are versions of I2c.h in which the 
flag is undefined.

At approximately line 262 of i2c-dev driver file i2c-dev.c, in function  
i2cdev_ioctl_wdrw(), this bit is set unconditionally, with the comment 
"memdup_user allocates with GFP_KERNEL, so DMA is ok ". This behavior is 
not the result of the userspace caller setting the flag, the driver sets 
it unconditionally. The result is that calls into i2c-dev using its 
ioctl() interface, as opposed to the read()/write() interface, always 
fail with error EINVAL.

Every version of i2c.h that I have defines I2C_M_DMA_SAFE, so the 
version of the nvidia driver that DKMS builds for me works. However, as 
the developer of ddcutil (https://github.com/rockowitz/ddcutil, 
https://www.ddcutil.com) I receive bug reports of failures with the 
nvidia proprietary driver due to this mismatch, so it is a problem.



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