Re: [PATCH v2 1/2] tpm: add retry logic

[Mimi Zohar <zohar@linux.vnet.ibm.com>]

On Mon, 2018-03-26 at 07:28 -0700, James Bottomley wrote:
> On Mon, 2018-03-26 at 19:41 +0530, Nayna Jain wrote:
> > 
> > On 03/22/2018 10:01 PM, James Bottomley wrote:
> > > 
> > > On Thu, 2018-03-22 at 21:43 +0530, Nayna Jain wrote:
> [...]
> > > > > +ssize_t tpm_transmit(struct tpm_chip *chip, struct tpm_space
> > > > > *space,
> > > > > +		     u8 *buf, size_t bufsiz, unsigned int
> > > > > flags)
> > > > > +{
> > > > > +	struct tpm_output_header *header = (struct
> > > > > tpm_output_header *)buf;
> > > > > +	/* space for header and handles */
> > > > > +	u8 save[TPM_HEADER_SIZE + 3*sizeof(u32)];
> > > > > +	unsigned int delay_msec = TPM2_DURATION_SHORT;
> > > > > +	u32 rc = 0;
> > > > > +	ssize_t ret;
> > > > > +	const size_t save_size = min(space ? sizeof(save) :
> > > > > TPM_HEADER_SIZE,
> > > > > +				     bufsiz);
> > > > > +
> > > > > +	/*
> > > > > +	 * Subtlety here: if we have a space, the handles will
> > > > > be
> > > > > +	 * transformed, so when we restore the header we also
> > > > > have
> > > > > to
> > > > > +	 * restore the handles.
> > > > > +	 */
> > > > > +	memcpy(save, buf, save_size);
> > > > > +
> > > > > +	for (;;) {
> > > > > +		ret = tpm_try_transmit(chip, space, buf,
> > > > > bufsiz,
> > > > > flags);
> > > > > +		if (ret < 0)
> > > > > +			break;
> > > > > +		rc = be32_to_cpu(header->return_code);
> > > > > +		if (rc != TPM2_RC_RETRY)
> > > > > +			break;
> > > > > +		delay_msec *= 2;
> > > > Was wondering if this increment could be moved after
> > > > tpm_msleep(delay_msec) ?
> > > I thought about that when I saw the logic in the original but what
> > > it would do is double the maximum delay (which is already nearly
> > > double TPM2_DURATION_LONG).  I figured doubling the initial timeout
> > > was fine rather than trying to do complex logic to get the delay
> > > exact (and then having to litter the file with comments explaining
> > > why).
> > 
> > Sorry James, I didn't understand exactly about what complex logic is 
> > involved. Can you please explain it more ?
> > My point was to just move "delay_msec *= 2"  after tpm_msleep() as
> > shown below:
> > 
> > if (delay_msec > TPM2_DURATION_LONG) {
> >      dev_err(&chip->dev, "TPM is in retry loop\n");
> >      break;
> > }
> > tpm_msleep(delay_msec)
> > delay_msec *= 2;
> 
> Yes, I understand the suggestion; however, if you simply do that,
> you'll end up with a useless sleep at the end of the wait period before
> you check to see if you've waited too long, which is even more
> suboptimal than the current situation.  The cardinal rule is we should
> only sleep if we know we're going to retry.

By the time delay_msec is incremented here, you've already finished
sleeping and will resend the command.  This is the reason I assume you
didn't use a normal for loop:

    for (delay_msec = TPM2_DURATION_SHORT; delay_msec <TPM2_DURATION_LONG;
         delay_msec *= 2)

Incrementing the sleep here will only affect the next sleep.

Mimi