Hi Arnd, On 06/11/18 6:21 PM, Arnd Bergmann wrote: > On 11/5/18, Kishon Vijay Abraham I <kishon@xxxxxx> wrote: >> On 05/11/18 8:46 AM, Chunyan Zhang wrote: >>> >>> + sdhci_switch_extdma(host, true); >> >> A number of devices using sdhci-omap supports ADMA. So switching to >> external >> DMA shouldn't be unconditional. >> >> IMHO sdhci.c should see if the device supports ADMA or SDMA. If not it >> should >> try switching to external DMA and if external DMA too is not supported, it >> should use PIO. > > What's the reasoning for preferring ADMA/SDMA over external DMA if > both are supported? Generally from our experiments we've found ADMA gives better throughput than DMA. I have to ascertain the actual reasons for that. > > I'd expect that if the external DMA for some reason is worse than > ADMA, we just wouldn't list it in the DT at all, but if it's listed and > ADMA also works, then the driver should try to use it before falling > back to ADMA. I would agree that for newer dtbs. However if you consider omap_hsmmc, external DMA bindings are already added in dt. If we try to switch to sdhci-omap with the same bindings, systems with older dtbs will use external DMA and give lesser throughput. Thanks Kishon
