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

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Message-ID: <20220824181754.GA2794837@bhelgaas>
Date:   Wed, 24 Aug 2022 13:17:54 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Serge Semin <fancer.lancer@...il.com>
Cc:     Serge Semin <Sergey.Semin@...kalelectronics.ru>,
        Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
        Vinod Koul <vkoul@...nel.org>, Rob Herring <robh@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Jingoo Han <jingoohan1@...il.com>, Frank Li <Frank.Li@....com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Pavel Parkhomenko <Pavel.Parkhomenko@...kalelectronics.ru>,
        linux-pci@...r.kernel.org, dmaengine@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v5 24/24] PCI: dwc: Add DW eDMA engine support

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, and not
something synthesized by the host bridge when it handles an
Unsupported Request completion, I'm fine with keeping it as is.