RE: [PATCH 2/2 v4] drm/exynos: added userptr feature.

[Inki Dae <inki.dae@samsung.com>]

ccing linux-mm

> -----Original Message-----
> From: KOSAKI Motohiro [mailto:kosaki.motohiro@gmail.com]
> Sent: Monday, May 14, 2012 4:05 PM
> To: Inki Dae
> Cc: 'KOSAKI Motohiro'; airlied@linux.ie; dri-devel@lists.freedesktop.org;
> j.glisse@gmail.com; minchan@kernel.org; kyungmin.park@samsung.com;
> sw0312.kim@samsung.com; jy0922.shim@samsung.com
> Subject: Re: [PATCH 2/2 v4] drm/exynos: added userptr feature.
> 
> (5/14/12 2:52 AM), Inki Dae wrote:
> >
> >
> >> -----Original Message-----
> >> From: KOSAKI Motohiro [mailto:kosaki.motohiro@gmail.com]
> >> Sent: Monday, May 14, 2012 3:33 PM
> >> To: Inki Dae
> >> Cc: airlied@linux.ie; dri-devel@lists.freedesktop.org;
> j.glisse@gmail.com;
> >> minchan@kernel.org; kyungmin.park@samsung.com; sw0312.kim@samsung.com;
> >> jy0922.shim@samsung.com
> >> Subject: Re: [PATCH 2/2 v4] drm/exynos: added userptr feature.
> >>
> >>> +       npages = buf->size>>  PAGE_SHIFT;
> >>
> >> Why round down? usually we use round up.
> >>
> >
> > The size was already rounded up by exynos_drm_gem_userptr_ioctl so this
> is
> > just used to get page count.
> 
> got it.
> 
> 
> 
> >>> +       down_read(&current->mm->mmap_sem);
> >>> +
> >>> +       /*
> >>> +        * Basically, all the pages from get_user_pages() can not be
not
> >> only
> >>> +        * migrated by CMA but also swapped out.
> >>> +        *
> >>> +        * The migration issue.
> >>> +        * - Pages reserved by CMA for some device using DMA could be
> >> used by
> >>> +        * kernel and if the device driver wants to use those pages
> >>> +        * while being used by kernel then the pages are copied into
> >>> +        * other ones allocated to migrate them and then finally,
> >>> +        * the device driver can use the pages for itself.
> >>> +        * Thus, migrated, the pages being accessed by DMA could be
> >> changed
> >>> +        * to other so this situation may incur that DMA accesses any
> >> pages
> >>> +        * it doesn't want.
> >>> +        *
> >>> +        * But the use of get_user_pages is safe from such magration
> >> issue
> >>> +        * because all the pages from get_user_pages CAN NOT be not
only
> >>> +        * migrated, but also swapped out.
> >>> +        */
> >>> +       get_npages = get_user_pages(current, current->mm, userptr,
> >>> +                                       npages, write, 1, buf->pages,
> > NULL);
> >>
> >> Why force=1? It is almostly core-dump specific option. Why don't you
> >> return
> >
> > I know that force indicates whether to force write access even  if user
> > mapping is readonly.
> 
> right. and then, usually we don't want to ignore access permission. but
> note,
> I'm only talk about generic thing. I have no knowledge drm area.
> 
> 
> 
> > so we just want to use pages from get_user_pages as
> > read/write permission.
> 
> >> EFAULT when the page has write permission. IOW, Why your Xorg module
> >> don't map memory w/ PROT_WRITE?
> >
> > No, Xorg can map memory w/ PROT_WRITE. Couldn't the Xorg map w/
> PROT_WRITE
> > if force = 1? plz, let me know if there is my missing point.
> 
> I meant, if Xorg always use PROT_WRITE, you don't need force=1.
> 


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