Re: [PATCH RESEND v5 24/24] PCI: dwc: Add DW eDMA engine support

[Serge Semin <fancer.lancer@gmail.com>]

On Wed, Aug 24, 2022 at 01:17:54PM -0500, Bjorn Helgaas wrote:
> On Wed, Aug 24, 2022 at 09:13:19PM +0300, Serge Semin wrote:
> > On Wed, Aug 24, 2022 at 11:51:18AM -0500, Bjorn Helgaas wrote:
> > > On Mon, Aug 22, 2022 at 09:53:32PM +0300, Serge Semin wrote:
> 
> > > > +	val = dw_pcie_readl_dbi(pci, PCIE_DMA_VIEWPORT_BASE + PCIE_DMA_CTRL);
> > > > +	if (val == 0xFFFFFFFF && pci->edma.reg_base) {
> > > > +		pci->edma.mf = EDMA_MF_EDMA_UNROLL;
> > > > +
> > > > +		val = dw_pcie_readl_dma(pci, PCIE_DMA_CTRL);
> > > > +	} else if (val != 0xFFFFFFFF) {
> > > 
> > 
> > > Consider PCI_POSSIBLE_ERROR() as an annotation about the meaning of
> > > 0xFFFFFFFF and something to grep for.
> > 
> > In this case FFs don't mean an error but a special value, which
> > indicates that the eDMA is mapped via the unrolled CSRs space. The
> > similar approach has been implemented for the iATU legacy/unroll setup
> > auto-detection. So I don't see much reasons to have it grepped, so as
> > to have a macro-based parametrization since the special value will
> > unluckily change while having the explicit literal utilized gives a
> > better understanding of the way the algorithm works.
> 

> If 0xFFFFFFFF is the result of a successful PCIe Memory Read,

Right. It is.

> and not
> something synthesized by the host bridge when it handles an
> Unsupported Request completion,

No it isn't. To be clear 0xFFs don't indicate some PCIe bus/controller
malfunction, but they are a result of reading the
DMA_CTRL_VIEWPORT_OFF register which doesn't exist. The manual
explicitly says: "Note - When register does not exist, value is fixed
to 32'hFFFF_FFFF". The register doesn't exist if either eDMA is
unavailable or the eDMA CSRs are mapped via the unrolled state. That
logic is used to auto-detect the eDMA availability and the way of it's
CSRs mapping.

> I'm fine with keeping it as is.

Ok. Thanks.

-Sergey