Re: [PATCH v2 2/6] dt-bindings: dma: dw: Add max burst transaction length property

[Rob Herring]

On Sun, May 17, 2020 at 08:47:39PM +0300, Serge Semin wrote:
> On Fri, May 15, 2020 at 02:11:13PM +0300, Serge Semin wrote:
> > On Fri, May 15, 2020 at 04:26:58PM +0530, Vinod Koul wrote:
> > > On 15-05-20, 13:51, Andy Shevchenko wrote:
> > > > On Fri, May 15, 2020 at 11:39:11AM +0530, Vinod Koul wrote:
> > > > > On 12-05-20, 15:38, Andy Shevchenko wrote:
> > > > > > On Tue, May 12, 2020 at 02:49:46PM +0300, Serge Semin wrote:
> > > > > > > On Tue, May 12, 2020 at 12:08:04PM +0300, Andy Shevchenko wrote:
> > > > > > > > On Tue, May 12, 2020 at 12:35:31AM +0300, Serge Semin wrote:
> > > > > > > > > On Tue, May 12, 2020 at 12:01:38AM +0300, Andy Shevchenko wrote:
> > > > > > > > > > On Mon, May 11, 2020 at 11:05:28PM +0300, Serge Semin wrote:
> > > > > > > > > > > On Fri, May 08, 2020 at 02:12:42PM +0300, Andy Shevchenko wrote:
> > > > > > > > > > > > On Fri, May 08, 2020 at 01:53:00PM +0300, Serge Semin wrote:
> > > > 
> > > > ...
> > > > 
> > > > > > I leave it to Rob and Vinod.
> > > > > > It won't break our case, so, feel free with your approach.
> > > > > 
> > > > > I agree the DT is about describing the hardware and looks like value of
> > > > > 1 is not allowed. If allowed it should be added..
> > > > 
> > > > It's allowed at *run time*, it's illegal in *pre-silicon stage* when
> > > > synthesizing the IP.
> > > 
> > > Then it should be added ..
> > 
> > Vinod, max-burst-len is "MAXimum" burst length not "run-time or current or any
> > other" burst length. It's a constant defined at the IP-core synthesis stage and
> > according to the Data Book, MAX burst length can't be 1. The allowed values are
> > exactly as I described in the binding [4, 8, 16, 32, ...]. MAX burst length
> > defines the upper limit of the run-time burst length. So setting it to 1 isn't
> > about describing a hardware, but using DT for the software convenience.
> > 
> > -Sergey
> 
> Vinod, to make this completely clear. According to the DW DMAC data book:
> - In general, run-time parameter of the DMA transaction burst length (set in
>   the SRC_MSIZE/DST_MSIZE fields of the channel control register) may belong
>   to the set [1, 4, 8, 16, 32, 64, 128, 256].
> - Actual upper limit of the burst length run-time parameter is limited by a
>   constant defined at the IP-synthesize stage (it's called DMAH_CHx_MAX_MULT_SIZE)
>   and this constant belongs to the set [4, 8, 16, 32, 64, 128, 256]. (See, no 1
>   in this set).
> 
> So the run-time burst length in a case of particular DW DMA controller belongs
> to the range:
> 1 <= SRC_MSIZE <= DMAH_CHx_MAX_MULT_SIZE
> and
> 1 <= DST_MSIZE <= DMAH_CHx_MAX_MULT_SIZE
> 
> See. No mater which DW DMA controller we get each of them will at least support
> the burst length of 1 and 4 transfer words. This is determined by design of the
> DW DMA controller IP since DMAH_CHx_MAX_MULT_SIZE constant set starts with 4.
> 
> In this patch I suggest to add the max-burst-len property, which specifies
> the upper limit for the run-time burst length. Since the maximum burst length
> capable to be set to the SRC_MSIZE/DST_MSIZE fields of the DMA channel control
> register is determined by the DMAH_CHx_MAX_MULT_SIZE constant (which can't be 1
> by the DW DMA IP design), max-burst-len property as being also responsible for
> the maximum burst length setting should be associated with DMAH_CHx_MAX_MULT_SIZE
> thus should belong to the same set [4, 8, 16, 32, 64, 128, 256].
> 
> So 1 shouldn't be in the enum of the max-burst-len property constraint, because
> hardware doesn't support such limitation by design, while setting 1 as
> max-burst-len would mean incorrect description of the DMA controller.
> 
> Vinod, could you take a look at the info I provided above and say your final word
> whether 1 should be really allowed to be in the max-burst-len enum constraints?
> I'll do as you say in the next version of the patchset.

I generally think the synthesis time IP configuration should be implied 
by the compatible string which is why we have SoC specific compatible 
strings (Of course I dream for IP vendors to make all that discoverable 
which is only occasionally the case). There are exceptions to this. If 
one SoC has the same IP configured in different ways, then we'd probably 
have properties for the differences.

As to whether h/w configuration is okay in DT, the answer is yes. The 
question is whether it is determined by SoC, board, OS and also who 
would set it and how often. Something tuned per board and independent of 
the OS/user is the ideal example. 

Rob