On Tue, Sep 17, 2019 at 03:03:29PM +0100, Robin Murphy wrote: > On 17/09/2019 14:49, Russell King - ARM Linux admin wrote: > > As already replied, v4 mode is not documented as being available on > > the LX2160A - the bit in the control register is marked as "reserved". > > This is as expected as it is documented that it is using a v3.00 of > > the SDHCI standard, rather than v4.00. > > > > So, sorry, enabling "v4 mode" isn't a workaround in this scenario. > > > > Given that v4 mode is not mandatory, this shouldn't be a work-around. > > > > Given that it _does_ work some of the time with the table >4GB, then > > this is not an addressing limitation. > > Yes, that's what "something totally different" usually means. > > > > However, the other difference between getting a single page directly from > > > the page allocator vs. the CMA area is that accesses to the linear mapping > > > of the CMA area are probably pretty rare, whereas for the single-page case > > > it's much more likely that kernel tasks using adjacent pages could lead to > > > prefetching of the descriptor page's cacheable alias. That could certainly > > > explain how reverting that commit manages to hide an apparent coherency > > > issue. > > > > Right, so how do we fix this? > > By describing the hardware correctly in the DT. It would appear that it _is_ correctly described given the default hardware configuration, but the driver sets a bit in a control register that enables cache snooping. Adding "dma-coherent" to the DT description does not seem to be the correct solution, as we are reliant on the DT description and driver implementation both agreeing, which is fragile. >From what I can see, there isn't a way for a driver to say "I've made this device is coherent now" and I suspect making the driver set the DMA snoop bit depending on whether "dma-coherent" is present in DT or not will cause data-corrupting regressions for other people. So, we're back to where we started - what is the right solution to this problem? The only thing I can think is that the driver needs to do something like: WARN_ON(!dev_is_dma_coherent(dev)); in esdhc_of_enable_dma() as a first step, and ensuring that the snoop bit matches the state of dev_is_dma_coherent(dev)? Is it permitted to use dev_is_dma_coherent() in drivers - it doesn't seem to be part of the normal DMA API? -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up
