linux-kernel - Re: [PATCH 4/4] sample/reserved_mem: Introduce a sample of struct page and dio support to no-map rmem

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Date:   Tue, 12 Jul 2022 08:26:15 +0800
From:   Li Chen <me@...ux.beauty>
To:     "Arnd Bergmann" <arnd@...db.de>
Cc:     "Catalin Marinas" <catalin.marinas@....com>,
        "Will Deacon" <will@...nel.org>,
        "Rob Herring" <robh+dt@...nel.org>,
        "Frank Rowand" <frowand.list@...il.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Li Chen" <lchen@...arella.com>,
        "Linux ARM" <linux-arm-kernel@...ts.infradead.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "DTML" <devicetree@...r.kernel.org>,
        "Linux-MM" <linux-mm@...ck.org>
Subject: Re: [PATCH 4/4] sample/reserved_mem: Introduce a sample of struct
 page and dio support to no-map rmem

Hi Arnd,
 ---- On Mon, 11 Jul 2022 21:28:10 +0800  Arnd Bergmann <arnd@...db.de> wrote --- 
 > On Mon, Jul 11, 2022 at 2:24 PM Li Chen <me@...ux.beauty> wrote:
 > >
 > > From: Li Chen <lchen@...arella.com>
 > >
 > > This sample driver shows how to build struct pages support to no-map rmem.
 > >
 > > Signed-off-by: Li Chen <lchen@...arella.com>
 > 
 > Not sure what a sample driver helps here if there are no actual users in-tree.
 > 
 > It would make more sense to merge the driver that wants to actually use this
 > first, and then add the additional feature.

Totally agree, but we plan to start rewriting our video driver in a long time, it has many legacy codes and I need to rewrite a lot of codes to migrate to v4l2.
That's why I also submit a sample driver here: to make the review progress easier and don't need reviewers to read video driver codes.

 > > +/*
 > > + * dts example
 > > + * rmem: rmem@1 {
 > > + *                     compatible = "shared-dma-pool";
 > > + *                     no-map;
 > > + *                     size = <0x0 0x20000000>;
 > > + *             };
 > > + * perf {
 > > + *             compatible = "example,rmem";
 > > + *             memory-region = <&rmem>;
 > > + *     };
 > 
 > The problem here is that the DT is meant to describe the platform in an OS
 > independent way, so having a binding that just corresponds to a user space
 > interface is not a good abstraction.

Gotcha, but IMO dts + rmem is the only choice for our use case. In our real case, we use reg instead of size to specify the physical address, so memremap cannot be used.

 > 
 > > +       vaddr = reserved_mem_memremap_pages(dev, rmem);
 > > +       if (IS_ERR_OR_NULL(vaddr))
 > > +               return PTR_ERR(vaddr);
 > 
 >  Using IS_ERR_OR_NULL() is usually an indication of a bad interface.
 > 
 > For the reserved_mem_memremap_pages(), you should decide whether to return
 > NULL on error or an error pointer, but not both.

Thanks, will fix in v2.

 > 
 >        Arnd
 >