linux-kernel - Re: [PATCH v9 2/3] drm: bridge: Add support for Cadence MHDP8546 DPI/DP bridge

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Date:   Thu, 3 Sep 2020 10:23:39 +0300
From:   Tomi Valkeinen <tomi.valkeinen@...com>
To:     Milind Parab <mparab@...ence.com>,
        Swapnil Kashinath Jakhade <sjakhade@...ence.com>,
        "airlied@...ux.ie" <airlied@...ux.ie>,
        "daniel@...ll.ch" <daniel@...ll.ch>,
        "Laurent.pinchart@...asonboard.com" 
        <Laurent.pinchart@...asonboard.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "a.hajda@...sung.com" <a.hajda@...sung.com>,
        "narmstrong@...libre.com" <narmstrong@...libre.com>,
        "jonas@...boo.se" <jonas@...boo.se>,
        "jernej.skrabec@...l.net" <jernej.skrabec@...l.net>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Yuti Suresh Amonkar <yamonkar@...ence.com>,
        "jsarha@...com" <jsarha@...com>, "nsekhar@...com" <nsekhar@...com>,
        "praneeth@...com" <praneeth@...com>,
        "nikhil.nd@...com" <nikhil.nd@...com>
Subject: Re: [PATCH v9 2/3] drm: bridge: Add support for Cadence MHDP8546
 DPI/DP bridge

Hi Milind,

On 03/09/2020 09:22, Milind Parab wrote:

> Also, note that CDNS MHDP implements DP_FRAMER_TU_p where bits 5:0 is tu_valid_symbols. So max programmable value is 63.
> Register document gives following explanation 
> "Number of valid symbols per Transfer Unit (TU). Rounded down to lower integer value (Allowed values are 1 to (TU_size-1)"
> 
> So, it says in case vs calculates to 64 (where Avail BW and Req BW are same) we program tu_valid_symbols = 63

Hmm, so "Rounded down to lower integer value" means

floor(x) - 1 ?

If that's the case, we need to subtract 1 in all cases, not only when req bw == avail bw.

> Third, is about the line_threshold calculation
> Unlike TU_SIZE and Valid_Symbols, line_threshold is implementation dependent
> 
> CDNS MHDP register specs gives the definition as " Video FIFO latency threshold" 
> Bits 5:0, Name "cfg_active_line_tresh", Description "Video Fifo Latency threshold. Defines the number of FIFO rows before reading starts. This setting depends on the transmitted video format and link rate."
> 
> This parameter is the Threshold of the FIFO. For optimal performance (considering equal write and read clock) we normally put the threshold in the mid of the FIFO.
> Hence the reset value is fixed as 32.
> Since symbol FIFO is accessed by Pxl clock and Symbol Link Clock the Threshold is set to a value which is dependent on the ratio of these clocks
> 
> line_threshold = full_fifo - fifo_ratio_due_to_clock_diff + 2
> where,
> full_fifo = (vs+1) * (8/bpp)
> fifo_ratio_due_to_clock_diff = ((vs+1) * pxlclock/mhdp->link.rate - 1) / mhdp->link.num_lanes 
> 
> Note that line_threshold can take a max value of 63

That doesn't result in anything sensible. 8/bpp is always 0.

 Tomi

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