RE: [Patch V3 1/3] tpm_tis-spi: Support hardware wait polling

[Krishna Yarlagadda <kyarlagadda@xxxxxxxxxx>]

> -----Original Message-----
> From: Mark Brown <broonie@xxxxxxxxxx>
> Sent: 24 February 2023 00:13
> To: Krishna Yarlagadda <kyarlagadda@xxxxxxxxxx>
> Cc: robh+dt@xxxxxxxxxx; peterhuewe@xxxxxx; jgg@xxxxxxxx;
> jarkko@xxxxxxxxxx; krzysztof.kozlowski+dt@xxxxxxxxxx; linux-
> spi@xxxxxxxxxxxxxxx; linux-tegra@xxxxxxxxxxxxxxx; linux-
> integrity@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> thierry.reding@xxxxxxxxx; Jonathan Hunter <jonathanh@xxxxxxxxxx>;
> Sowjanya Komatineni <skomatineni@xxxxxxxxxx>; Laxman Dewangan
> <ldewangan@xxxxxxxxxx>
> Subject: Re: [Patch V3 1/3] tpm_tis-spi: Support hardware wait polling
> 
> On Thu, Feb 23, 2023 at 06:41:43PM +0000, Krishna Yarlagadda wrote:
> 
> > > > +       spi_bus_lock(phy->spi_device->master);
> > > > +
> > > > +       while (len) {
> 
> > > Why?
> 
> > TPM supports max 64B in single transaction. Loop to send rest of it.
> 
> No, why is there a bus lock?
Bus lock to avoid other clients to be accessed between TPM transfers.

> 
> > > > +		spi_xfer[0].tx_buf = phy->iobuf;
> > > > +		spi_xfer[0].len = 1;
> > > > +		spi_message_add_tail(&spi_xfer[0], &m);
> > > > +
> > > > +		spi_xfer[1].tx_buf = phy->iobuf + 1;
> > > > +		spi_xfer[1].len = 3;
> > > > +		spi_message_add_tail(&spi_xfer[1], &m);
> 
> > > Why would we make these two separate transfers?
> 
> > Tegra QSPI combined sequence requires cmd, addr and data in different
> > transfers. This eliminates need for additional flag for combined sequence.
> 
> That needs some documentation, and we might need a flag to ensure the
> core doesn't coalesce the transfers.
Will add comment at top of the function. Bus lock should avoid coalesce of
transfer of single message from others.
KY