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Date:   Thu, 25 Aug 2022 08:16:14 +0300
From:   Serge Semin <fancer.lancer@gmail.com>
To:     Bjorn Helgaas <helgaas@kernel.org>
Cc:     Serge Semin <Sergey.Semin@baikalelectronics.ru>,
        Gustavo Pimentel <gustavo.pimentel@synopsys.com>,
        Vinod Koul <vkoul@kernel.org>, Rob Herring <robh@kernel.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        Jingoo Han <jingoohan1@gmail.com>, Frank Li <Frank.Li@nxp.com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>,
        Lorenzo Pieralisi <lpieralisi@kernel.org>,
        Krzysztof =?utf-8?Q?Wilczy=C5=84ski?= <kw@linux.com>,
        Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>,
        Pavel Parkhomenko <Pavel.Parkhomenko@baikalelectronics.ru>,
        linux-pci@vger.kernel.org, dmaengine@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH RESEND v5 24/24] PCI: dwc: Add DW eDMA engine support
Message-ID: <20220825051614.kfify5fbqlhurvdn@mobilestation>
References: <20220824181319.wkj4256a5jp2xjlp@mobilestation>
 <20220824181754.GA2794837@bhelgaas>
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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