linux-kernel - Re: [PATCH 09/15] device core: Add ability to handle multiple dma offsets

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Message-ID: <20200520140320.GA3624154@kroah.com>
Date:   Wed, 20 May 2020 16:03:20 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Jim Quinlan <james.quinlan@...adcom.com>
Cc:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Saravana Kannan <saravanak@...gle.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE" 
        <devicetree@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DMA MAPPING HELPERS" <iommu@...ts.linux-foundation.org>
Subject: Re: [PATCH 09/15] device core: Add ability to handle multiple dma
 offsets

On Wed, May 20, 2020 at 09:50:36AM -0400, Jim Quinlan wrote:
> On Wed, May 20, 2020 at 1:43 AM Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> wrote:
> >
> > On Tue, May 19, 2020 at 04:34:07PM -0400, Jim Quinlan wrote:
> > > diff --git a/include/linux/device.h b/include/linux/device.h
> > > index ac8e37cd716a..6cd916860b5f 100644
> > > --- a/include/linux/device.h
> > > +++ b/include/linux/device.h
> > > @@ -493,6 +493,8 @@ struct dev_links_info {
> > >   * @bus_dma_limit: Limit of an upstream bridge or bus which imposes a smaller
> > >   *           DMA limit than the device itself supports.
> > >   * @dma_pfn_offset: offset of DMA memory range relatively of RAM
> > > + * @dma_map: Like dma_pfn_offset but used when there are multiple
> > > + *           pfn offsets for multiple dma-ranges.
> > >   * @dma_parms:       A low level driver may set these to teach IOMMU code about
> > >   *           segment limitations.
> > >   * @dma_pools:       Dma pools (if dma'ble device).
> > > @@ -578,7 +580,12 @@ struct device {
> > >                                            allocations such descriptors. */
> > >       u64             bus_dma_limit;  /* upstream dma constraint */
> > >       unsigned long   dma_pfn_offset;
> > > -
> > > +#ifdef CONFIG_DMA_PFN_OFFSET_MAP
> > > +     const void *dma_offset_map;     /* Like dma_pfn_offset, but for
> > > +                                      * the unlikely case of multiple
> > > +                                      * offsets. If non-null, dma_pfn_offset
> > > +                                      * will be 0. */
> > > +#endif
> > >       struct device_dma_parameters *dma_parms;
> > >
> > >       struct list_head        dma_pools;      /* dma pools (if dma'ble) */
> >
> > I'll defer to Christoph here, but I thought we were trying to get rid of
> > stuff like this from struct device, not add new things to it for dma
> Hi Greg,
> 
> I wasn't aware of this policy.  I put it in 'struct device' because
> just like the existing dma_pfn_offset; it seemed to be the only way to
> pull this off.  I'll certainly follow any ideas on alternative
> strategies from Christoph et al.
> 
> > apis.  And why is it a void *?
> Just wanted to minimize the number of lines I've added to device.h, no
> other reason.

How would using a real type make this more lines?  Never use a void *
unless you have to, we want the compiler to check our errors for us :)

thanks,

greg k-h