On 10/22/24 17:38, Niklas Cassel wrote: > On Tue, Oct 22, 2024 at 10:51:53AM +0900, Damien Le Moal wrote: >> On 10/22/24 07:19, Bjorn Helgaas wrote: >>> On Sat, Oct 12, 2024 at 08:32:40PM +0900, Damien Le Moal wrote: >> >> DMA transfers that can be done using dedicated DMA rx/tx channels associated >> with the endpoint controller do not need to use this API as the mapping to >> the host PCI address space is automatically handled by the DMA driver. > > I disagree with this part. It think that it depends on the DMA controller. > > Looking at the manual for e.g. the embedded DMA controller on the DWC > controller (eDMA): > "" > When you do not want the iATU to translate outbound requests that are generated by the > internal DMA module, then you must implement one of the following approaches: > - Ensure that the combination of DMA channel programming and iATU control register > programming, causes no translation of DMA traffic to be done in the iATU. > or > - Activate the DMA bypass mode to allow request TLPs which are initiated by the DMA > controller to pass through the iATU untranslated. You can activate DMA bypass mode by > setting the DMA Bypass field of the iATU Control 2 Register (IATU_REGION_C- > TRL_OFF_2_OUTBOUND_0). > "" > > However, we also know that, if there is no match in the iATU table: > "" > The default behavior of the ATU when there is no address match in the outbound direction or no > TLP attribute match in the inbound direction, is to pass the transaction through. > "" > > I.e. if there is a iATU mapping (which is what pci_epc_map_addr() sets up), > the eDMA will take that into account. If there is none, it will go through > untranslated. > > > So for the eDMA embedded on the DWC controller, we do not strictly need to > call pci_epc_map_addr(). (However, we probably want to do it anyway, as long > as we haven't enabled DMA bypass mode, just to make sure that there is no > other iATU mapping in the mapping table that will affect our transfer.) > > However, if you think about a generic DMA controller, e.g. an ARM primecell > pl330, I don't see how it that DMA controller will be able to perform > transfers correctly if there is not an iATU mapping for the region that it > is reading/writing to. > > So the safest thing, that will work with any DMA controller is probably to > always call pci_epc_map_addr() before doing the transfer. Indeed, I think you are right. Not doing the mapping before DMA with the RK3588 works fine, but that may not be the case for all controllers. -- Damien Le Moal Western Digital Research
