linux-kernel - Re: [For Linux 4/4] xen/displif: add ABI for para-virtual display

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Message-ID: <82ade2ce-93e0-d817-b289-ca09fa1bb66e@arm.com>
Date:   Fri, 7 Apr 2017 14:50:57 +0100
From:   Julien Grall <julien.grall@....com>
To:     Oleksandr Andrushchenko <andr2000@...il.com>,
        xen-devel@...ts.xenproject.org
Cc:     joculator@...il.com, al1img@...il.com, vlad.babchuk@...il.com,
        andrii.anisov@...il.com, olekstysh@...il.com,
        konrad.wilk@...cle.com, lars.kurth@...rix.com,
        sstabellini@...nel.org, linux-kernel@...r.kernel.org,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>,
        Oleksandr Grytsov <oleksandr_grytsov@...m.com>
Subject: Re: [For Linux 4/4] xen/displif: add ABI for para-virtual display

Hi Oleksandr,

On 07/04/17 09:30, Oleksandr Andrushchenko wrote:
> +/*
> + ******************************************************************************
> + *                        Back to front events delivery
> + ******************************************************************************
> + * In order to deliver asynchronous events from back to front a shared page is
> + * allocated by front and its granted reference propagated to back via
> + * XenStore entries (evt-ring-ref/evt-event-channel).
> + * This page has a common header used by both front and back to synchronize
> + * access and control event's ring buffer, while back being a producer of the
> + * events and front being a consumer. The rest of the page after the header
> + * is used for event packets.
> + *
> + * Upon reception of an event(s) front may confirm its reception
> + * for either each event, group of events or none.
> + */
> +
> +struct xendispl_event_page {
> +	uint32_t in_cons;
> +	uint32_t in_prod;
> +	uint8_t reserved[56];
> +};
> +
> +#define XENDISPL_EVENT_PAGE_SIZE 4096

This will be always the size of a grant (e.g xen page size), right? If 
so, I would prefer if you use XEN_PAGE_SIZE to so we can easily update 
Linux in the case Xen is using a different page size.

Note that the Front-end, Backend and Xen may use different page size, so 
your drivers would need to cope with that. The current approach is to 
always use the page granularity of Xen.

> +#define XENDISPL_IN_RING_OFFS (sizeof(struct xendispl_event_page))
> +#define XENDISPL_IN_RING_SIZE (XENDISPL_EVENT_PAGE_SIZE - XENDISPL_IN_RING_OFFS)
> +#define XENDISPL_IN_RING_LEN (XENDISPL_IN_RING_SIZE / sizeof(struct xendispl_evt))
> +#define XENDISPL_IN_RING(page) \
> +	((struct xendispl_evt *)((char *)(page) + XENDISPL_IN_RING_OFFS))
> +#define XENDISPL_IN_RING_REF(page, idx) \
> +	(XENDISPL_IN_RING((page))[(idx) % XENDISPL_IN_RING_LEN])
> +
> +#endif /* __XEN_PUBLIC_IO_DISPLIF_H__ */
> +
> +/*
> + * Local variables:
> + * mode: C
> + * c-file-style: "BSD"
> + * c-basic-offset: 4
> + * tab-width: 4
> + * indent-tabs-mode: nil
> + * End:
> + */
>

Cheers,

-- 
Julien Grall