Re: [PATCH] tpm_tis_spi: fix:release chip select when flow control fails

[shaopeijie]

On Sat, 22 Apr 2023 00:46:40 +0300, jarkko@xxxxxxxxxx wrote:
> On Fri, Mar 31, 2023 at 02:56:25PM +0800, shaopeijie@xxxxxxxx wrote:
>> From: Peijie Shao <shaopeijie@xxxxxxxx>
>> 
>> The TPM's chip select will leave active after spi_bus_unlock when
>> flow control timeout, and may interfere other chips sharing the same
>> spi bus, or may damage them dule to level conflict on MISO pin.
>> 
>> So the patch deactives the chip select by sending an empty message
>> with cs_change=0 if flow control fails.
>> 
>> The reason why flow control timeout for me is unfortunately I got a
>> damaged TPM chip. It always pull MISO low during cs active(this can
>> be easily emulated by wire MISO to the ground), not responding anything,
>> and dmesg shows below:
>> ...
>> [   311.150725] tpm_tis_spi: probe of spi0.0 failed with error -110
>> ...
>
> We don't really cease to support damaged hardware but it is true
> that the *software* failure paths should probably be robust enough
> to deativate chip select.
>
> I would rewrite this as
>
> "The failure paths in tpm_tis_spi_transfer() do not deactivate
> chip select. Send an empty message (cs_select == 0) to overcome
> this."
>
> That's all there needs to be. We do not care about broken hardware.
>

I agree. The patch is to robust *software* path, but not support 
broken hardware.

>> 
>> Signed-off-by: Peijie Shao <shaopeijie@xxxxxxxx>
>> ---
>>  drivers/char/tpm/tpm_tis_spi_main.c | 13 ++++++++++++-
>>  1 file changed, 12 insertions(+), 1 deletion(-)
>> 
>> diff --git a/drivers/char/tpm/tpm_tis_spi_main.c b/drivers/char/tpm/tpm_tis_spi_main.c
>> index a0963a3e92bd..5c8ff343761f 100644
>> --- a/drivers/char/tpm/tpm_tis_spi_main.c
>> +++ b/drivers/char/tpm/tpm_tis_spi_main.c
>> @@ -105,8 +105,19 @@ int tpm_tis_spi_transfer(struct tpm_tis_data *data, u32 addr, u16 len,
>>  		/* Flow control transfers are receive only */
>>  		spi_xfer.tx_buf = NULL;
>>  		ret = phy->flow_control(phy, &spi_xfer);
>> -		if (ret < 0)
>> +		if (ret < 0) {
>> +			/*
>> +			 * Release cs pin if the device is not responding, regardless of the reason.
>> +			 * Notice cs may alreadly been released if the failure was caused inside
>> +			 * spi_sync_locked called by flow_control, in this situation, a pluse may be
>> +			 * generated on cs.
>> +			 */
>
> Please replace above comment with:
>
> /* Deactivate chip select: */
>

I agree.

>> +			memset(&spi_xfer, 0, sizeof(spi_xfer));
>> +			spi_message_init(&m);
>> +			spi_message_add_tail(&spi_xfer, &m);
>> +			spi_sync_locked(phy->spi_device, &m);
>>  			goto exit;
>> +		}
>>  
>>  		spi_xfer.cs_change = 0;
>>  		spi_xfer.len = transfer_len;
>> -- 
>> 2.39.1
>> 
>> 
>> 
>
> There's three call sites, why are you taking care of only one
> of them?
>
> I'd consider instead:
>
>        return 0;
>
> exit:
>        memset(&spi_xfer, 0, sizeof(spi_xfer));
>        spi_message_init(&m);
>        spi_message_add_tail(&spi_xfer, &m);
>        spi_sync_locked(phy->spi_device, &m);
>        spi_bus_unlock(phy->spi_device->master);
>        return ret;
> }

I found that spi_sync_locked() will deactivate cs if any failure
is generated inside it.

    spi_sync_locked()
	...
        spi_transfer_one_message()
            ...
            if (ret != 0 || !keep_cs)
                spi_set_cs(msg->spi, false, false);

spi_transfer_one_message() is the default transfer method, I think 
other spi controllers who implements .transfer_one_message should 
have the same behaviour.

Sending an empty message when cs is already deactivated may have a small
side effect: cs will go activate and deactivate in a very short time,
means a pulse will be generated on cs pin. This may also happen when
failure is generated by spi_sync_locked() which called inside ->flow_control().

So to reduce this, I prefer deactivating cs only when phy->flow_control()
fails. If the side effect is totaly acceptable, your advice maybe better.

Waiting for your suggestions.

>
> The the rollback would apply to all call sites.
>
> BR, Jarkko

Thanks!

Peijie, Shao