linux-kernel - Re: [PATCH v4 3/3] iommu: dart: Add DART iommu driver

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Date:   Mon, 12 Jul 2021 13:02:24 +0200
From:   "Sven Peter" <sven@...npeter.dev>
To:     "Alyssa Rosenzweig" <alyssa@...labora.com>
Cc:     "Will Deacon" <will@...nel.org>,
        "Robin Murphy" <robin.murphy@....com>,
        "Joerg Roedel" <joro@...tes.org>,
        "Arnd Bergmann" <arnd@...nel.org>, devicetree@...r.kernel.org,
        "Hector Martin" <marcan@...can.st>, linux-kernel@...r.kernel.org,
        "Marc Zyngier" <maz@...nel.org>,
        "Mohamed Mediouni" <mohamed.mediouni@...amail.com>,
        "Stan Skowronek" <stan@...ellium.com>,
        linux-arm-kernel@...ts.infradead.org,
        "Mark Kettenis" <mark.kettenis@...all.nl>,
        "Petr Mladek via iommu" <iommu@...ts.linux-foundation.org>,
        "Alexander Graf" <graf@...zon.com>,
        "Alyssa Rosenzweig" <alyssa.rosenzweig@...labora.com>,
        "Rob Herring" <robh+dt@...nel.org>,
        "Rouven Czerwinski" <r.czerwinski@...gutronix.de>
Subject: Re: [PATCH v4 3/3] iommu: dart: Add DART iommu driver

Hi,


On Wed, Jun 30, 2021, at 15:49, Alyssa Rosenzweig wrote:
> Looks really good! Just a few minor comments. With them addressed,
> 
> 	Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>

Thanks!

> 
> > +	  Say Y here if you are using an Apple SoC with a DART IOMMU.
> 
> Nit: Do we need to spell out "with a DART IOMMU"? Don't all the apple
> socs need DART?

Good point, I'll remove it.

> 
> > +/*
> > + * This structure is used to identify a single stream attached to a domain.
> > + * It's used as a list inside that domain to be able to attach multiple
> > + * streams to a single domain. Since multiple devices can use a single stream
> > + * it additionally keeps track of how many devices are represented by this
> > + * stream. Once that number reaches zero it is detached from the IOMMU domain
> > + * and all translations from this stream are disabled.
> > + *
> > + * @dart: DART instance to which this stream belongs
> > + * @sid: stream id within the DART instance
> > + * @num_devices: count of devices attached to this stream
> > + * @stream_head: list head for the next stream
> > + */
> > +struct apple_dart_stream {
> > +	struct apple_dart *dart;
> > +	u32 sid;
> > +
> > +	u32 num_devices;
> > +
> > +	struct list_head stream_head;
> > +};
> 
> It wasn't obvious to me why we can get away without reference counting.
> Looking ahead it looks like we assert locks in each case. Maybe add
> that to the comment?

Sure, I'll add that to the comment.

> 
> ```
> > +static void apple_dart_hw_set_ttbr(struct apple_dart *dart, u16 sid, u16 idx,
> > +				   phys_addr_t paddr)
> > +{
> > +	writel(DART_TTBR_VALID | (paddr >> DART_TTBR_SHIFT),
> > +	       dart->regs + DART_TTBR(sid, idx));
> > +}
> ```
> 
> Should we be checking alignment here? Something like
> 
>     BUG_ON(paddr & ((1 << DART_TTBR_SHIFT) - 1));
> 

Sure, right now paddr will always be aligned but adding that
BUG_ON doesn't hurt :)



Best,

Sven