linux-kernel - Re: [PATCH] usb: dwc3: host: inherit dma configuration from parent dev

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Date:   Wed, 07 Sep 2016 13:24:07 +0300
From:   Felipe Balbi <balbi@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>, Peter Chen <hzpeterchen@...il.com>
Cc:     Leo Li <pku.leo@...il.com>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Russell King - ARM Linux <linux@....linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        "linux-usb\@vger.kernel.org" <linux-usb@...r.kernel.org>,
        Sekhar Nori <nsekhar@...com>,
        lkml <linux-kernel@...r.kernel.org>,
        Stuart Yoder <stuart.yoder@....com>,
        Scott Wood <oss@...error.net>,
        David Fisher <david.fisher1@...opsys.com>,
        "Thang Q. Nguyen" <tqnguyen@....com>,
        Alan Stern <stern@...land.harvard.edu>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-arm-kernel\@lists.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] usb: dwc3: host: inherit dma configuration from parent dev


Hi,

Arnd Bergmann <arnd@...db.de> writes:

[...]

> Regarding the DMA configuration that you mention in ci_hdrc_add_device(),
> I think we should replace 
>
>         pdev->dev.dma_mask = dev->dma_mask;
>         pdev->dev.dma_parms = dev->dma_parms;
>         dma_set_coherent_mask(&pdev->dev, dev->coherent_dma_mask);
>
> with of_dma_configure(), which has the chance to configure more than
> just those three, as the dma API might look into different aspects:
>
> - iommu specific configuration
> - cache coherency information
> - bus type
> - dma offset
> - dma_map_ops pointer
>
> We try to handle everything in of_dma_configure() at configuration
> time, and that would be the place to add anything else that we might
> need in the future.

There are a couple problems with this:

1) won't work for PCI-based systems.

DWC3 is used in production PCI-based HW and also in Synopsys HAPS DX
platform (FPGA that appears like a PCI card to host PC)


2) not very robust solution

of_dma_configure() will hardcode 32-bit DMA dmask for xhci-plat because
that's not created by DT. The only reason why this works at all is
because of the default 32-bit dma mask thing :-) So, how is it any
different than copying 32-bit dma mask from parent?

-- 
balbi

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