Messages in this thread | | | Date | Fri, 7 Nov 2014 10:29:39 -0800 | From | Scott Branden <> | Subject | Re: [PATCHv2 3/5] mmc: shdci-bcm2835: add efficient back-to-back write workaround |
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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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