[Xcb] How can I increase the maximum request length?

[Bart Massey]

Uli: Thanks for looking at this more closely and confirming my earlier
analysis!

> Worse, it cannot (easily) support splitting up huge requests. It returns
the void cookie that is used by the request. If it generates multiple
requests, which of the cookies should be returned?

Could maybe just force all but the last cookie and return that. Return the
error cookie if one of the earlier requests fails. Loses the async, but is
call-compatible.

Probably better to add another entry point that returns all the cookies as
needed.

On Wed, Jan 30, 2019 at 4:31 AM Uli Schlachter <psychon at znc.in> wrote:

> Hi,
>
> On 19.01.19 03:51, paul ewing wrote:
> > Ah, I think the previous email I linked to may have simply been wrong. It
> > states that BIG-REQUESTS increases the max request size to 16GB from
> 256KB but
> > I think the maximum is actually only 16MB.
>
> The maximum request length is determined by the X11 server. Without
> BIG-REQUESTS, the X11 server can announce support for at most 256 KiB,
> but it can also support less. BIG-REQUESTS only changes this so that the
> X11 server *can* support bigger requests. However, the limit is still
> imposed by the X11 server and any overly large requests cause the
> connection to be terminated. So no, you cannot increase the limit.
>
> Also, XCB automatically uses the BIG-REQUESTS extension to query the
> maximum supported request size. So, you can just use
> xcb_get_maximum_request_length() to query the limit that the X11 server
> announces. Splitting huge requests into smaller chunks is something that
> libxcb does not do for you.
>
> > I'll split up the image in the application code for now and see if I can
> come
> > up with a clean implementation before submitting a pull request to xcb.
>
> Possibly helpful pointer:
>
> https://gitlab.freedesktop.org/cairo/cairo/commit/51435eeb0c4d9e53d76dae8b5af279ad518a05f8
>
> > ________________________________
> > From: Bart Massey <bart at cs.pdx.edu>
> > Sent: Friday, January 18, 2019 5:39 PM
> > To: paul ewing
> > Cc: xcb at lists.freedesktop.org
> > Subject: Re: [Xcb] How can I increase the maximum request length?
> >
> > Offhand, looks like your bigger display is too big to use a single
> PutImage request even with BIG-REQUESTS enabled. Looks like xcb_image_put()
> doesn't break the request into pieces for you (maybe it should? pull
> requests welcome) so you'll have to put the image a piece at a time: break
> it into bands and use the y offset.
>
> From a quick look: xcb_image_put() does not break requests into smaller
> chunks. It just calls xcb_put_image() with some values it got out of the
> xcb_image_t*.
>
> Worse, it cannot (easily) support splitting up huge requests. It returns
> the void cookie that is used by the request. If it generates multiple
> requests, which of the cookies should be returned?
>
>
> https://sources.debian.org/src/xcb-util-image/0.4.0-1/image/xcb_image.c/?hl=448#L448
>
> Cheers,
> Uli
> --
> Homophobia - The fear that another man will treat you the way you treat
> women.
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