I would be very happy if someone with access to
the documentation could shed some further light on this.

According to the SH7757 manual, setting the CLKDIV[3:0] in the CE_CLK_CTRL register 0xF selects Pck as the bus clock frequency. So, unlike other devices where Pck/2 (CLKDIV[3:0]=0) is the max bus speed, the SH7757 can run at Pck when CLKDIV=0xF.

- Chris


-----Original Message-----
From: linux-sh-owner@vger.kernel.org [mailto:linux-sh-owner@vger.kernel.org] On Behalf Of Simon Horman
Sent: Wednesday, June 20, 2012 3:02 AM
To: Guennadi Liakhovetski
Cc: linux-mmc@vger.kernel.org; linux-sh@vger.kernel.org; Chris Ball; Magnus Damm
Subject: Re: [PATCH] mmc: sh_mmcif: mmc->f_max heuristic

On Wed, Jun 20, 2012 at 08:49:38AM +0200, Guennadi Liakhovetski wrote:

Hi Simon

Thanks for addressing this issue.

On Wed, 20 Jun 2012, Simon Horman wrote:

quoted

In 930f152cc9998388031af577843baae572ac8ab6 ("mmc: sh_mmcif: mmc->f_max
should be half of the bus clock") I changed the setting of mmc->f_max from
the bus clock to half the bus clock based on the manual for the sh7372 SoC.

Inspection of sh_mmcif_clock_control() reveals that it relies on
mmc->f_max being set to the bus speed in order to enable the supplementary
clock, a feature that does not exist on the sh7372.

Armed with this information implement the following heuristic for setting
mmc->f_max:

* Use bus clock if the supplementary clock feature is present
  - Assumed to work on the sh7757lcr board, the only board present
    in the tree which has the feature.

To be able to better understand this change: do we have access to the 
sh7757 documentation and does it actually explain how the CLK_SUP_PCLK bit 
in MMCIF_CE_CLK_CTRL functions? Does it actually set the MMC bus clock to 
be equal to the host clock?

My understanding is that neither Magnus nor I have access to the
documentation for the sh7757. Thus we could only infer things from
examining the source code. I would be very happy if someone with access to
the documentation could shed some further light on this. In particular,
I would be happy if this heuristic was shown not to be necessary. But
in lieu of access to the documentation I think this patch is reasonable.

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