Re: [PATCH 3/4] i2c: i801: Improve i801_block_transaction_byte_by_byte

Heiner Kallweit
2023-08-29T06:08:47Z

On 28.08.2023 17:10, Jean Delvare wrote:
> On Mon, 28 Aug 2023 15:27:47 +0200, Jean Delvare wrote:
>> On Sun, 27 Aug 2023 19:14:38 +0200, Heiner Kallweit wrote:
>>> We set SMBHSTCNT_LAST_BYTE whilst the host is receiving the last byte.
>>> Apparently the host checks for SMBHSTCNT_LAST_BYTE once it received
>>> a byte, in order to determine whether to ack the byte or not.
>>> So SMBHSTCNT_LAST_BYTE doesn't have to be set before the host starts
>>> receiving the last byte.
>>
>> How is this not racy?
>>
>> In the interrupt-driven case, at the end of a block read transaction,
>> we set SMBHSTCNT_LAST_BYTE at the end of i801_isr_byte_done(), then
>> return to i801_isr() where we write 1 to SMBHSTSTS_BYTE_DONE to clear
>> it. This lets the controller handle the last byte with the knowledge
>> that this is the last byte.
>>
>> However, in the poll-driven case, SMBHSTSTS_BYTE_DONE is being cleared
>> at the end of the loop in i801_block_transaction_byte_by_byte(), then
>> at the beginning of the next iteration, we write SMBHSTCNT_LAST_BYTE,
>> then wait for completion. If the controller is super fast (or, to be
>> more realistic, the i2c-i801 driver gets preempted between writing
>> SMBHSTSTS_BYTE_DONE and writing SMBHSTCNT_LAST_BYTE) then the byte may
>> have been already read and acked, before we have the time to let the
>> controller know that no ACK should be sent. This looks racy. Am I
>> missing something?
>
> I made a little experiment which, I think, proves my point.
>
> Firstly, I loaded the i2c-i801 driver with disable_features=0x12, to
> make sure the poll-based byte-by-byte code path is used. The EEPROM
> data starts with:
>
> 0 1 2 3 4 5 6 7 8 9 a b c d e f
> 00: 92 13 0b 02 04 21 02 01 03 52 01 08 0a 00 fc 00
>
> The following commands read the first 4 bytes with an I2C Block Read
> command, then fetch the next byte from the EEPROM (without specifying
> the offset):
>
> # /usr/local/sbin/i2cget -y 4 0x53 0 i 4 ; /usr/local/sbin/i2cget -y 4 0x53
> 0x92 0x13 0x0b 0x02
> 0x04
>
> As you can see, I get the 5 first bytes of the EEPROM, as expected.
>
> Then I added an arbitrary delay in the driver where I think the race
> condition exists:
>
> --- a/drivers/i2c/busses/i2c-i801.c
> +++ b/drivers/i2c/busses/i2c-i801.c
> @@ -684,8 +684,10 @@ static int i801_block_transaction_byte_b
>
> if (i == 1)
> outb_p(smbcmd | SMBHSTCNT_START, SMBHSTCNT(priv));
> - else if (smbcmd & SMBHSTCNT_LAST_BYTE)
> + else if (smbcmd & SMBHSTCNT_LAST_BYTE) {
> + usleep_range(10000, 20000);
> outb_p(smbcmd, SMBHSTCNT(priv));
> + }
>
> status = i801_wait_byte_done(priv);
> if (status)
>
> I loaded the modified i2c-i801 driver, still with
> disable_features=0x12. Running the same commands again, I now get:
>
> # /usr/local/sbin/i2cget -y 4 0x53 0 i 4 ; /usr/local/sbin/i2cget -y 4 0x53
> 0x92 0x13 0x0b 0x02
> 0x21
>
> So I get EEPROM bytes 0-3 then it jumps to offset 5 directly. This
> means that the EEPROM started sending the byte at offset 4 at the end
> of the I2C Block Read transfer, due to the controller sending an ACK
> after the byte at offset 3.
>
> For the code to be safe, we need to set SMBHSTCNT_LAST_BYTE *before*
> clearing SMBHSTSTS_BYTE_DONE.
>

I think you're right with the preemption scenario. I'll submit a fix
for it.

> Note: the transfers do not fail, only the internal register pointer of
> the EEPROM gets screwed, so this is probably not an issue for devices
> which don't rely on an internal register pointer, only EEPROM-like
> devices are affected.
>