Hi Thierry, > From: Thierry Reding, Sent: Tuesday, September 10, 2019 4:19 AM > > On Mon, Sep 09, 2019 at 06:13:31PM +0200, Christoph Hellwig wrote: > > On Mon, Sep 09, 2019 at 02:56:56PM +0200, Thierry Reding wrote: > > > From: Thierry Reding <treding@xxxxxxxxxx> > > > > > > When enabling the DMA map merging capability for a queue, ensure that > > > the maximum segment size does not exceed the device's limit. > > > > We can't do that unfortunately. If we use the virt_boundary setting > > we do aggressive merges that there is no accounting for. So we can't > > limit the segment size. > > But that's kind of the point here. My understanding is that the maximum > segment size in the device's DMA parameters defines the maximum size of > the segment that the device can handle. > > In the particular case that I'm trying to fix, according to the SDHCI > specification, these devices can handle segments that are a maximum of > 64 KiB in size. If we allow that segment size to be exceeded, the device > will no longer be able to handle them. > > > And at least for the case how devices usually do the addressing > > (page based on not sgl based) that needs the virt_boundary there isn't > > really any concept like a segment anyway. > > I do understand that aspect of it. However, devices that do the > addressing this way, wouldn't they want to set the maximum segment size > to something large (like UINT_MAX, which many users that don't have the > concept of a segment already do)? > > If you disagree, do you have any alternative proposals other than > reverting the offending patch? linux-next is currently broken on all > systems where the SDHCI controller is behind an IOMMU. I'm sorry for causing this trouble on your environment. I have a proposal to resolve this issue. The mmc_host struct will have a new caps2 flag like MMC_CAP2_MERGE_CAPABLE and add a condition into the queue.c like below. + if (host->caps2 & MMC_CAP2_MERGE_CAPABLE && + host->max_segs < MMC_DMA_MAP_MERGE_SEGMENTS && dma_get_merge_boundary(mmc_dev(host))) host->can_dma_map_merge = 1; else host->can_dma_map_merge = 0; After that, all mmc controllers disable the feature as default, and if a mmc controller has such capable, the host driver should set the flag. Ulf, is such a patch acceptable for v5.4-rcN? Anyway, I'll submit such a patch later. Best regards, Yoshihiro Shimoda > Thierry
