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

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Date:	Mon, 17 Mar 2014 10:07:46 +0800
From:	Axel Lin <axel.lin@...ics.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	linux-spi@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH] spi: sc18is602: Don't be that restrictive with the
 maximum transfer speed

2014-03-17 9:47 GMT+08:00 Guenter Roeck <linux@...ck-us.net>:
> 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.

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.

Regards,
Axel
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