linux-kernel - Re: [PATCH v7 23/25] PCI: dwc: Restore DMA-mask after MSI-data allocation

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Date:   Fri, 16 Dec 2022 13:18:27 +0300
From:   Serge Semin <fancer.lancer@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Robin Murphy <robin.murphy@....com>,
        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>,
        Cai Huoqing <cai.huoqing@...ux.dev>,
        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>,
        caihuoqing <caihuoqing@...du.com>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        linux-pci@...r.kernel.org, dmaengine@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 23/25] PCI: dwc: Restore DMA-mask after MSI-data
 allocation

On Fri, Dec 16, 2022 at 01:49:38AM -0800, Christoph Hellwig wrote:
> On Fri, Dec 16, 2022 at 12:34:23PM +0300, Serge Semin wrote:
> > What about instead of save/restore pattern I'll just change the
> > dma_set_mask_and_coherent() method with the dma_set_coherent_mask()
> > function call? It seems cleaner. Like this:
> 
> > Thus the platform-specific streaming DMA mask would be preserved.
> > Since it's PCIe then having the streaming DMA-mask less than 32-bits
> > wide is very much improbable. Moreover DW PCIe AXI-interface can be
> > synthesize only with one out of two address bus widths: 32 and 64.
> 

> Where platform-specific means the dwc subdriver? 

Right. I meant the streaming DMA-mask set by the low-level DWC PCIe drivers
(like pcie-qcom(-ep)?.c, pcie-bt1.c, etc). It's very much important to
have the real DMA-mask (at least the streaming one) set for the eDMA-capable
controllers so the DMA-engine clients would work with the best performance.

> Yes, that seems to work. 

Ok. I'll just use the direct dma_set_coherent_mask() method here then.

> Alternatively have a flag that says which streaming mask
> to set.

I'd prefer to have more flexibility here relying on the low-level
drivers to set the mask(s) instead of adding the new flag, just in case
if there is vendor-specific IP-core/platform changes in the address
bus width.

-Serge(y)