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

[Scott Branden]

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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