linux-kernel - Re: [PATCH v2 1/4] tpm: Use a threaded interrupt handler

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Date:   Thu, 29 Apr 2021 00:37:40 +0200
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     peterhuewe@....de, jgg@...pe.ca, stefanb@...ux.vnet.ibm.com,
        James.Bottomley@...senpartnership.com, keescook@...omium.org,
        jsnitsel@...hat.com, ml.linux@...oe.vision,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] tpm: Use a threaded interrupt handler


Hi,

On 28.04.21 at 01:53, Jarkko Sakkinen wrote:
> On Mon, Apr 26, 2021 at 01:47:17AM +0200, Lino Sanfilippo wrote:
>> Interrupt handling at least includes reading and writing the interrupt
>> status register from the interrupt routine. However over SPI those accesses
>> require a sleepable context, since a mutex is used in the concerning
>> functions.
>> For this reason request a threaded interrupt handler which is running in
>> (sleepable) process context.
>>
>> Signed-off-by: Lino Sanfilippo <LinoSanfilippo@....de>
>> ---
>>  drivers/char/tpm/tpm_tis_core.c | 6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/char/tpm/tpm_tis_core.c b/drivers/char/tpm/tpm_tis_core.c
>> index e7d1eab..0959559 100644
>> --- a/drivers/char/tpm/tpm_tis_core.c
>> +++ b/drivers/char/tpm/tpm_tis_core.c
>> @@ -781,8 +781,10 @@ static int tpm_tis_probe_irq_single(struct tpm_chip *chip, u32 intmask,
>>  	int rc;
>>  	u32 int_status;
>>
>> -	if (devm_request_irq(chip->dev.parent, irq, tis_int_handler, flags,
>> -			     dev_name(&chip->dev), chip) != 0) {
>> +
>> +	if (devm_request_threaded_irq(chip->dev.parent, irq, NULL,
>> +				      tis_int_handler, IRQF_ONESHOT | flags,
>> +				      dev_name(&chip->dev), chip) != 0) {
>>  		dev_info(&chip->dev, "Unable to request irq: %d for probe\n",
>>  			 irq);
>>  		return -1;
>> --
>> 2.7.4
>>
>
> Why?
>
> https://elixir.bootlin.com/linux/v5.12/source/drivers/char/tpm/tpm_tis_core.c#L670
>
> I don't see anything that sleeps there.
>
> /Jarkko1
>

The problem are the register read/write functions which we use to access the status register in
the interrupt handler. In case of SPI they result in taking the spi_bus_lock which is a mutex.

E.g tpm_tis_spi_read32: tpm_tis_spi_read_bytes->tpm_tis_spi_transfer->spi_bus_lock->mutex_lock

Using a threaded interrupt handler seemed to me the easiest way to avoid this issue.

Regards,
Lino