Re: [PATCH 08/11] MTD: m25p80: Add option to limit SPI transfer size.

Marek Vasut
2015-07-15T12:37:14Z

On Wednesday, July 15, 2015 at 11:45:07 AM, Michal Suchanek wrote:
> On 4 June 2015 at 19:15, Richard Cochran <richardcochran@gmail.com> wrote:
> > On Thu, Jun 04, 2015 at 10:31:45AM +0200, Michal Suchanek wrote:
> >> You might want to try to run the bus at 60MHz or 80MHz and then the
> >> values would probably again be different.
> >>
> >> The first two values are set in DT so the logical place for setting
> >> the third is also in DT.
> >>
> >> Otherwise you would need some in-kernel table of these settings.
> >
> > Or a formula.
>
> This formula probably needs to take into account
>
> - the unknown reason for the pl330 to fail transfer

Shouldn't that be fixed at the PL330 level ? This looks like fixing
a problem at the wrong place :)

> - the device transfer speed and transfer phase as set in DT
> - possibly device-specific latency and board-specific trace design
> and assembly tolerances

If the design is broken, then cap the speed as for normal SPI device.

> Seriously, until I have at least a vague idea why the transfer fails I
> am not comfortable pulling some formula out of thin air and pretending
> I have a working patch.
>
> On the other hand, a parameter you can set in the DT and which comes
> with a suggested value which can be tuned depending on the system
> seems more viable.

The problem is, if you add a new DT binding, you'd have to support it
forever, no matter how bad idea that binding turned out to be.

Best regards,
Marek Vasut