On 9 November 2018 at 06:27, Kishon Vijay Abraham I <kishon@xxxxxx> wrote: > 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. I was under the impression that the internal DMA (for old versions) is rather poor. However, in the end this is a software policy decision of what to use. I have no strong opinion, so feel free to pick what you think makes best sense. Maybe Adrian also have some thoughts? Kind regards Uffe
