RE: [Patch V6 1/3] spi: Add TPM HW flow flag

[Krishna Yarlagadda <kyarlagadda@xxxxxxxxxx>]

> -----Original Message-----
> From: Thierry Reding <thierry.reding@xxxxxxxxx>
> Sent: 01 March 2023 19:04
> To: Krishna Yarlagadda <kyarlagadda@xxxxxxxxxx>
> Cc: robh+dt@xxxxxxxxxx; broonie@xxxxxxxxxx; peterhuewe@xxxxxx;
> jgg@xxxxxxxx; jarkko@xxxxxxxxxx; krzysztof.kozlowski+dt@xxxxxxxxxx; linux-
> spi@xxxxxxxxxxxxxxx; linux-tegra@xxxxxxxxxxxxxxx; linux-
> integrity@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Jonathan Hunter
> <jonathanh@xxxxxxxxxx>; Sowjanya Komatineni
> <skomatineni@xxxxxxxxxx>; Laxman Dewangan <ldewangan@xxxxxxxxxx>
> Subject: Re: [Patch V6 1/3] spi: Add TPM HW flow flag
> 
> On Mon, Feb 27, 2023 at 10:51:06PM +0530, Krishna Yarlagadda wrote:
> > TPM spec defines flow control over SPI. Client device can insert a wait
> 
> Maybe add a reference to where in the TPM specification this can be
> found? It looks like the specifications are publicly available, though
> I'm less sure about stability of the links, so perhaps it's enough to
> name the document and section that this can be found in. QEMU seems to
> be using this link to point to the specification, which I suppose has a
> good chance of remaining stable:
> 
> 	https://trustedcomputinggroup.org/resource/pc-client-work-group-
> pc-client-specific-tpm-interface-specification-tis/
> 
> It looks like the latest version is 1.3 revision 27 and the details of
> this flow control mechanism are in section "6.4.5. Flow Control".
> 
> > state on MISO when address is trasmitted by controller on MOSI. It can
> 
> "transmitted"
> 
> > work only on full duplex.
> > Half duplex controllers need to implement flow control in HW.
> 
> This is a bit confusing because you first say it will only work for full
> duplex controllers and then you say it's also possible for half-duplex
> controllers.
> 
> Maybe reword this to something like:
> 
> 	Detecting the wait state in software is only possible for full
> 	duplex controllers. For controllers that support only half-
> 	duplex, the wait state detection needs to be implemented in
> 	hardware.
> 
> > Add a flag for TPM to indicate flow control is expected in controller.
> 
> That's not exactly what the flag indicates, though, is it? It primarily
> indicates that the device uses TPM flow control. It's then up to the
> controller to configure itself accordingly (i.e. if it supports half-
> duplex, enable detection of the wait state, otherwise leave it up to the
> client driver to detect the wait state).
Flag is enabled only if controller is half-duplex and HW flow control is
needed. Will change wording to say it is enabled for HW flow control.
> 
> >
> > Signed-off-by: Krishna Yarlagadda <kyarlagadda@xxxxxxxxxx>
> > ---
> >  include/linux/spi/spi.h | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
> > index 4fa26b9a3572..6b32c90e9e20 100644
> > --- a/include/linux/spi/spi.h
> > +++ b/include/linux/spi/spi.h
> > @@ -184,8 +184,9 @@ struct spi_device {
> >  	u8			chip_select;
> >  	u8			bits_per_word;
> >  	bool			rt;
> > -#define SPI_NO_TX	BIT(31)		/* No transmit wire */
> > -#define SPI_NO_RX	BIT(30)		/* No receive wire */
> > +#define SPI_NO_TX		BIT(31)		/* No transmit wire */
> > +#define SPI_NO_RX		BIT(30)		/* No receive wire */
> > +#define SPI_TPM_HW_FLOW		BIT(29)		/* TPM flow
> control */
> 
> Maybe some (or all?) of the information in the commit message should be
> duplicated here? That way people wouldn't need to go look for the commit
> message in order to find out.
> 
> Given what I said above about the flag, it may be better to name this
> SPI_TPM_FLOW_CONTROL, but I suppose what you have here is fine, too.
> 
> Thierry