linux-kernel - Re: [PATCHv2 3/5] mmc: shdci-bcm2835: add efficient back-to-back write workaround

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Date:	Fri, 7 Nov 2014 10:29:39 -0800
From:	Scott Branden <sbranden@...adcom.com>
To:	Stephen Warren <swarren@...dotorg.org>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Russell King <rmk+kernel@....linux.org.uk>,
	"Peter Griffin" <peter.griffin@...aro.org>,
	Chris Ball <chris@...ntf.net>,
	"Piotr Krol" <pietrushnic@...il.com>
CC:	<linux-mmc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Joe Perches <joe@...ches.com>,
	<linux-rpi-kernel@...ts.infradead.org>,
	Ray Jui <rjui@...adcom.com>,
	<bcm-kernel-feedback-list@...adcom.com>
Subject: Re: [PATCHv2 3/5] mmc: shdci-bcm2835: add efficient back-to-back
 write workaround

On 14-11-05 08:48 PM, Stephen Warren wrote:
> On 11/04/2014 11:55 PM, Scott Branden wrote:
>> On 14-11-04 08:57 PM, Stephen Warren wrote:
>>> On 10/30/2014 12:36 AM, Scott Branden wrote:
>>>> The bcm2835 has clock domain issues when back to back writes to certain
>>>> registers are written.  The existing driver works around this issue with
>>>> udelay.  A more efficient method is to store the 8 and 16 bit writes
>>>> to the registers affected and then write them as 32 bits at the
>>>> appropriate
>>>> time.
>>>
>>>> diff --git a/drivers/mmc/host/sdhci-bcm2835.c
>>>> b/drivers/mmc/host/sdhci-bcm2835.c
>>>
>>>>    static void bcm2835_sdhci_writew(struct sdhci_host *host, u16 val,
>>>> int reg)
>>>>    {
>>>>        struct sdhci_pltfm_host *pltfm_host = sdhci_priv(host);
>>>> -    struct bcm2835_sdhci *bcm2835_host = pltfm_host->priv;
>>>> -    u32 oldval = (reg == SDHCI_COMMAND) ? bcm2835_host->shadow :
>>>> -        bcm2835_sdhci_readl(host, reg & ~3);
>>>> +    struct bcm2835_sdhci_host *bcm2835_host = pltfm_host->priv;
>>>
>>> Is that type change for bcm2835_host really correct?
>>
>> Yes - at the top of the patch the structure has been expanded and named
>> appropriately.
>>
>> -struct bcm2835_sdhci {
>> -    u32 shadow;
>> +struct bcm2835_sdhci_host {
>> +    u32 shadow_cmd;
>> +    u32 shadow_blk;
>>   };
>
> Ah yes, sorry for missing that.
>
>>>> +    } else {
>>>> +        /* Read reg, all other registers are not shadowed */
>>>> +        oldval = readl(host->ioaddr + (reg & ~3));
>>>
>>> Is there any reason to use readl() directly here rather than calling
>>> bcm2835_readl()? ...
>>
>> Yes, bcm2835_readl does not need to be called in read-modify-write and
>> shadow register situations and just adds overhead.  All that needs to be
>> called is readl.  bcm2835_readl has some existing ugly code in it to
>> modify the capabilities register on a read function.  This info never
>> needs to be for write as you can't overwrite the capabilities register.
>
> To be honest, it seems better to do all the read/write through
> consistent functions. One advantage of bcm2835_readl() is that it
> consistently adds on the base address internally so you don't have to
> write it out every time manually. Still, the code ought to work fine
> after this change, so I guess it's OK.
>
>> I hope to get rid of the capabilities hack in a future patch as this
>> should never have been acceptable in upstreamed code to begin with.  The
>> capabilities override should have been passed in through a device tree
>> entry.
>
> It's a pretty common technique with precedent. I certainly don't agree
> that it should be configured by DT. Arguably, DT makes sense to describe
> board-to-board variations, but there's almost zero point putting data
> into DT that is SoC description rather than board description; just put
> it into the driver to avoid continually parsing the same data over and
> over from DT just to get back to the same data that could have been
> encoded into the driver. If the data varies between similar controllers,
> an of_match table can easily be used to parameterize it based on
> compatible value.
There is work to be done here or I will be unable to use this driver in 
our chipsets.  Perhaps it will be easier having another driver 
actually... as the DMA seems quite different on RPI.
>

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