Re: [PATCH] spi: sc18is602: Don't be that restrictive with the maximum transfer speed

[Guenter Roeck]

On 03/16/2014 07:07 PM, Axel Lin wrote:
> 2014-03-17 9:47 GMT+08:00 Guenter Roeck <linux@xxxxxxxxxxxx>:
> > Commit 09e99bca8 (spi: sc18is602: Convert to let spi core validate
> > transfer speed) made the maximum transfer speed much more restrictive
> > than before. The transfer speed used to be adjusted to 1/4 of the chip
> > clock rate if a higher transfer speed was requested. Now such transfers are
> > simply rejected. With default settings, this causes, for example, a transfer
> > request at 2 mbps to be rejected because the maximum speed with the default
> > chip clock is 1.843 mbps.
> >
> > This is unnecessarily restrictive and causes unnecessary failures. Loosen
> > the limit to accept transfers up to 50% of the clock rate and adjust
> > the speed as needed when setting up the actualt transfer.
>
> I suppose this controller can only set to SC18IS602_MODE_CLOCK_DIV_4 for the
> highest transfer speed. If this is the case, master->max_speed_hz should be
> hw->freq / 4.

That really depends on one's point of view. The chip does not support a transfer
speed of, say, hw->freq / 5 or hw->freq / 6 either, but adjusts it to the next
available speed. Following your logic, every non-exact speed should be rejected,
which would make it a pain for a user to find a working speed.

> Now I'm thinking if it is ok to use master->max_speed_hz as transfer speed when
> xfer->speed_hz > master->max_speed_hz. And it should be handled in spi core.
> I'm sending a RFC patch now.
That is an acceptable alternate solution for me.

Guenter

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