linux-kernel - Re: [PATCH v7] Add udmabuf misc device

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Message-ID: <20180911065014.vo6qp6hkb7cjftdc@sirius.home.kraxel.org>
Date:   Tue, 11 Sep 2018 08:50:14 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     dri-devel@...ts.freedesktop.org, David Airlie <airlied@...ux.ie>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Jonathan Corbet <corbet@....net>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Shuah Khan <shuah@...nel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, linux-api@...r.kernel.org
Subject: Re: [PATCH v7] Add udmabuf misc device

  Hi,

> > +#define UDMABUF_CREATE       _IOW('u', 0x42, struct udmabuf_create)
> 
> Why do you start at 0x42 if you reserve the 0x40-0x4f range ?

No particular strong reason, just that using 42 was less boring than
starting with 0x40.

> > +#define UDMABUF_CREATE_LIST  _IOW('u', 0x43, struct udmabuf_create_list)
> 
> Where's the documentation ? :-)

Isn't it simple enough?

But, well, yes, I guess I can add some kerneldoc comments.

> > +static int udmabuf_vm_fault(struct vm_fault *vmf)
> > +{
> > +	struct vm_area_struct *vma = vmf->vma;
> > +	struct udmabuf *ubuf = vma->vm_private_data;
> > +
> > +	if (WARN_ON(vmf->pgoff >= ubuf->pagecount))
> > +		return VM_FAULT_SIGBUS;
> 
> Just curious, when do you expect this to happen ?

It should not.  If it actually happens it would be a bug somewhere,
thats why the WARN_ON.

> > +	struct udmabuf *ubuf;

> > +	ubuf = kzalloc(sizeof(struct udmabuf), GFP_KERNEL);
> 
> sizeof(*ubuf)

Why?  Should not make a difference ...

> > +		memfd = fget(list[i].memfd);
> > +		if (!memfd)
> > +			goto err_put_pages;
> > +		if (!shmem_mapping(file_inode(memfd)->i_mapping))
> > +			goto err_put_pages;
> > +		seals = memfd_fcntl(memfd, F_GET_SEALS, 0);
> > +		if (seals == -EINVAL ||
> > +		    (seals & SEALS_WANTED) != SEALS_WANTED ||
> > +		    (seals & SEALS_DENIED) != 0)
> > +			goto err_put_pages;
> 
> All these conditions will return -EINVAL. I'm not familiar with the memfd API, 
> should some error conditions return a different error code to make them 
> distinguishable by userspace ?

Hmm, I guess EBADFD would be reasonable in case the file handle isn't a
memfd.  Other suggestions?

I'll prepare a fixup patch series addressing most of the other
review comments.

cheers,
  Gerd