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Date: Sun, 9 Nov 1997 08:40:52 -0500
From: Randall Hopper <rhh@ct.picker.com>
To: Amancio Hasty <hasty@rah.star-gate.com>
Cc: multimedia@freebsd.org
Subject: bt848 bandwidth limitation
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     Speaking of bandwidth limitations, I noticed this slightly puzzling
check in the driver to determine when to split a scanline transfer into
halves in the RISC program:

     /* When to split a dma transfer , the bt848 has timing as well as
        dma transfer size limitations so that we have to split dma
        transfers into two dma requests 
        */
     #define DMA_BT848_SPLIT 319*2
    
      ...

      if ((width * pixel_width) < DMA_BT848_SPLIT ) {


To take an example, if we're doing 4Bpp 640x480, a scanline will be 2560
bytes.  This "( 640*4 < 638 )" check will of course trigger a scanline
split, but each half of the scanline is 1280 bytes -- over twice the value
of DMA_BT848_SPLIT.

So what's got me wondering is that this is such a small number and we only
ever split into 2 segments.   Thoughts?

Randall