Re: [PATCH 1/2] i2c: designware: Only check the first byte for SMBus block read length

On Mon, Jun 15, 2020 at 07:07:42PM +0300, Andy Shevchenko wrote:

On Mon, Jun 15, 2020 at 7:06 PM Sultan Alsawaf [off-list ref] wrote:

quoted

On Mon, Jun 15, 2020 at 12:40:19PM +0300, Andy Shevchenko wrote:

quoted

On Sun, Jun 14, 2020 at 02:02:54PM -0700, Sultan Alsawaf wrote:

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From: Sultan Alsawaf <redacted>

SMBus block reads can be broken because the read function will just skip
over bytes it doesn't like until reaching a byte that conforms to the
length restrictions for block reads. This is problematic when it isn't
known if the incoming payload is indeed a conforming block read.

According to the SMBus specification, block reads will only send the
payload length in the first byte, so we can fix this by only considering
the first byte in a sequence for block read length purposes.

I'm wondering if this overlaps with [1]. AFAIU that one is also makes sure that
the length is not a garbage.

[1]: https://lore.kernel.org/linux-i2c/20200613104109.2989-1-mans@mansr.com/T/#u (local)

No overlap.

Thanks for clarifying.

quoted

That looks like a similar bug for a different driver. In my case,
the adapter provides native SMBus support, so emulation is never used. This is
clear to see by looking at i2c_transfer_buffer_flags(), which only uses the
master_xfer functions provided by the adapter; it doesn't call the emulation
path at all.

But do we get an advantage if this can be done in the i2c core instead
(once for all)?

We can't, because the adapter driver needs to know mid-transfer to look for the
payload length in the first byte, and then alter the transfer size on-the-fly.
That can't be done in the i2c core, sadly. The problem is that we don't know if
a transfer is going to be a block read or not beforehand. And altering the
transfer size mid-transfer is definitely a controller specific task.

Sultan