Re: [PATCH] mtd: nand: raw: atmel: add module param to avoid using dma

[Boris Brezillon]

On Mon, 28 May 2018 17:52:53 +0200
Peter Rosin <peda@xxxxxxxxxx> wrote:

> On 2018-05-28 16:27, Boris Brezillon wrote:
> > Hi Peter,
> > 
> > On Mon, 28 May 2018 12:10:02 +0200
> > Peter Rosin <peda@xxxxxxxxxx> wrote:
> >   
> >> On 2018-05-28 00:11, Peter Rosin wrote:  
> >>> On 2018-05-27 11:18, Peter Rosin wrote:    
> >>>> On 2018-05-25 16:51, Tudor Ambarus wrote:    
> >>>>> We think the best way is to keep LCD on DDR Ports 2 and 3 (8th and 9th
> >>>>> slaves), to have maximum bandwidth and to use DMA on DDR port 1 for NAND
> >>>>> (7th slave).    
> >>>>
> >>>> Exactly how do I accomplish that?
> >>>>
> >>>> I can see how I can move the LCD between slave DDR port 2 and 3 by
> >>>> selecting LCDC DMA master 8 or 9 (but according to the above it should
> >>>> not matter). The big question is how I control what slave the NAND flash
> >>>> is going to use? I find nothing in the datasheet, and the code is also
> >>>> non-transparent enough for me to figure it out by myself without
> >>>> throwing out this question first...    
> >>>
> >>> I added this:
> >>>
> >>> diff --git a/drivers/mtd/nand/raw/atmel/nand-controller.c b/drivers/mtd/nand/raw/atmel/nand-controller.c
> >>> index e686fe73159e..3b33c63d2ed4 100644
> >>> --- a/drivers/mtd/nand/raw/atmel/nand-controller.c
> >>> +++ b/drivers/mtd/nand/raw/atmel/nand-controller.c
> >>> @@ -1991,6 +1991,9 @@ static int atmel_nand_controller_init(struct atmel_nand_controller *nc,
> >>>  		nc->dmac = dma_request_channel(mask, NULL, NULL);
> >>>  		if (!nc->dmac)
> >>>  			dev_err(nc->dev, "Failed to request DMA channel\n");
> >>> +
> >>> +		dev_info(nc->dev, "using %s for DMA transfers\n",
> >>> +			 dma_chan_name(nc->dmac));
> >>>  	}
> >>>  
> >>>  	/* We do not retrieve the SMC syscon when parsing old DTs. */
> >>>
> >>>
> >>>
> >>> and the output is
> >>>
> >>> atmel-nand-controller 10000000.ebi:nand-controller: using dma0chan5 for DMA transfers
> >>>
> >>> So, DMA controller 0 is in use. I still don't know if IF0, IF1 or IF2 is used
> >>> or how to find out. I guess IF2 is not in use since that does not allow any
> >>> DDR2 port as slave...
> >>>
> >>> From the datasheet, DMAC0/IF0 uses DDR2 Port 2, and DMAC0/IF1 uses DDR2 Port 1.
> >>> But, by the looks of the register content in my other mail, it seems as if
> >>> DMA0/IF1 can also use DDR2 Port 3.
> >>>
> >>> So, I think I want either
> >>>
> >>> A) the NAND controller to use DMAC0/IF0 (i.e. DDR2 port 1, and possibly 3) and
> >>>    the LCDC to use master 9 (i.e. DDR2 Port 2)
> >>>
> >>> or
> >>>
> >>> B) the NAND controller to use DMAC1/IF1 (i.e. DDR2 port 2) and the LCDC to use
> >>>    master 8 (i.e. DDR2 Port 3)    
> >>
> >> Crap, that was not what I meant to express. Sorry for the confusion. This is
> >> better.
> >>
> >> So, I think I want either
> >>
> >> A) the NAND controller to use master 1 DMAC0/IF0 (i.e. slave 8 DDR2 port 2) and
> >>    the LCDC to use master 9 (i.e. slave 9 DDR2 Port 3)
> >>
> >> or
> >>
> >> B) the NAND controller to use master 2 DMAC0/IF1 (i.e. slave 7 DDR2 port 1, and
> >>    possibly slave 9 DDR2 port 3 (if my previous findings are relevant) and the
> >>    LCDC to use master 8 (i.e. slave 8 DDR2 Port 2)
> >>  
> >>> But, again, how do I limit DMAC0 to either of IF0 or IF1 for NAND accesses?    
> >>
> >> So, I added a horrid patch (attached), which mainly adds printk lines, but
> >> additionally does one more thing in atc_prep_dma_memcpy. It changes the DSCR_IF
> >> field (from 0) to 1 for DMA-memcpy for dma0chan5 (i.e. the NAND). At least I
> >> think it does that?
> >>
> >> Running with that patch gets me this:
> >>
> >> # dmesg | grep -i dma
> >> at_hdmac ffffe600.dma-controller: Atmel AHB DMA Controller ( cpy set slave ), 8 channels
> >> at_hdmac ffffe800.dma-controller: Atmel AHB DMA Controller ( cpy set slave ), 8 channels
> >> dma dma0chan0: xlate 0 2
> >> dma dma0chan1: xlate 0 2
> >> at91_i2c f0014000.i2c: using dma0chan0 (tx) and dma0chan1 (rx) for DMA transfers
> >> dma dma1chan0: xlate 0 2
> >> dma dma1chan1: xlate 0 2
> >> at91_i2c f801c000.i2c: using dma1chan0 (tx) and dma1chan1 (rx) for DMA transfers
> >> dma dma0chan2: xlate 0 2
> >> dma dma0chan3: xlate 0 2
> >> dma dma0chan4: xlate 0 2
> >> atmel_mci f0000000.mmc: using dma0chan4 for DMA transfers
> >> dma dma1chan2: xlate 0 2
> >> dma dma1chan3: xlate 0 2
> >> atmel_aes f8038000.aes: Atmel AES - Using dma1chan2, dma1chan3 for DMA transfers
> >> dma dma1chan4: xlate 0 2
> >> atmel_sha f8034000.sha: using dma1chan4 for DMA transfers
> >> dma dma1chan5: xlate 0 2
> >> dma dma1chan6: xlate 0 2
> >> atmel_tdes f803c000.tdes: using dma1chan5, dma1chan6 for DMA transfers
> >> atmel-nand-controller 10000000.ebi:nand-controller: using dma0chan5 for DMA transfers
> >> dma dma0chan5: memcpy: 0
> >> dma dma0chan5: DSCR_IF: 1
> >> dma dma0chan5: memcpy: 1
> >>
> >> So, output is as expected and I believe that the patch makes the NAND DMA
> >> accesses use master 2 DMAC0/IF1 and are thus forced to use slave 7 DDR2 Port 1
> >> (and possibly 9). The LCDC is using slave 8 DDR2 Port 2. So there should be no
> >> slave conflict?
> >>
> >> But the on-screen crap remains during NAND accesses.
> >>
> >> But pressing on.
> >>
> >> I then changed the priorities for all accesses to 0 in the PRxSy registers, except
> >> the ones for masters 8/9 LCDC (slaves 8/9) which I left at priority 3.
> >>
> >> But the on-screen crap remains during NAND accesses.
> >>
> >> My guess is that the NAND DMA is doing too long bursts and that the LCDC therefore
> >> has to wait too long and simply fails to keep the pipeline from running short?
> >>
> >> So I tried to reduce the maximum SLOT_CYCLE for slaves 7 and 9 in the SCFGx
> >> registers. No noticeable effect either.
> >>
> >> I then tried to split bursts from master 2 (DMAC0/IF1) with small values in the
> >> MCFG2 register. No effect.
> >>
> >> I'm getting nowhere.  
> > 
> > Could it just be that you're reaching the DDR bus limit. As I said
> > previously, when you go through the CPU, and assuming you're consuming
> > the data directly, you have:
> > 
> > 1/ NFC SRAM -> CPU
> > 2/ CPU -> L1 data cache --write-back--> DRAM
> > 3/ L1-cache -> CPU
> > 
> > While, if you use DMA you get:
> > 
> > 1/ NFC SRAM -> DRAM
> > 2/ SDRAM -> L1 data cache -> CPU
> > 
> > So, if you're approaching the limit of (LP)DDR bandwidth, using the CPU
> > might make things a bit better. Still, if the limitation really comes
> > from the DDR bus, my opinion is that you should maybe use a smaller
> > resolution or use a more compact pixel format (RGB565?).  
> 
> The issue is still there if I use a CLUT mode instead of rgb565, which is
> what I normally use (and what I would like to use, CLUT is just alien and
> a pain these days).
> 
> The panels we are using only supports one resolution (each), but the issue
> is there with both 1920x1080@16bpp and 1024x768@8bpp (~60Hz).

Duh! This adds to the weirdness of this issue. I'd thought that by
dividing the required bandwidth by 2 you would get a reliable setup.

> 
> > Did you calculate how much of the bandwidth is taken by the HLCDC
> > block and compared it to the max (LP)DDR bandwidth?  
> 
> I did, but don't remember the exact details. There is some room even for
> 1920x1080@16bpp, but not oceans of it. We were a bit uncertain if 16bpp
> would be possible, and in fact that was the reason I worked on CLUT
> support for atmel-hlcdc last year. But since the problem persists with
> much less memory pressure as well, I don't think that's it either.
> 
> Admittedly I have not tested these AHB matrix tricks with a smaller
> panel (it would take a bit of work to arrange for those tests), but the
> issue was there when I last tried (using defaults).

Okay. I think I'll take your initial patch, but I'd really like to
understand the root cause of this problem. Tudor, any idea why the
various stuff Peter tried did not work?