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 11:52:56 +0000
From:	Mark Brown <broonie@...nel.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org,
	Axel Lin <axel.lin@...ics.com>
Subject: Re: [PATCH] spi: sc18is602: Don't be that restrictive with the
 maximum transfer speed

On Sun, Mar 16, 2014 at 06:47:50PM -0700, Guenter Roeck wrote:
> 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.

Given this description I'd expect to see a change in the core not a
driver - like the other fork of the thread said I'd expect to deal with
the issue by improving the constraint handling code.

>  	master->transfer_one_message = sc18is602_transfer_one;
>  	master->dev.of_node = np;
>  	master->min_speed_hz = hw->freq / 128;
> -	master->max_speed_hz = hw->freq / 4;
> +	master->max_speed_hz = hw->freq / 2;

That said, if this is something that the hardware can support it makes
sense to do it anyway - is there an actual spec constraint available?

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