linux-kernel - Re: [PATCH v5 2/2] spi: dw: Add dma controller capability checks

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Message-ID: <20230411144743.if72zn5et33khkcp@mobilestation>
Date:   Tue, 11 Apr 2023 17:47:43 +0300
From:   Serge Semin <fancer.lancer@...il.com>
To:     Andy Shevchenko <andriy.shevchenko@...el.com>
Cc:     Joy Chakraborty <joychakr@...gle.com>,
        Mark Brown <broonie@...nel.org>, linux-spi@...r.kernel.org,
        linux-kernel@...r.kernel.org, manugautam@...gle.com,
        rohitner@...gle.com
Subject: Re: [PATCH v5 2/2] spi: dw: Add dma controller capability checks

On Tue, Apr 11, 2023 at 05:38:01PM +0300, Serge Semin wrote:
> On Tue, Apr 11, 2023 at 03:18:34PM +0300, Andy Shevchenko wrote:
> > On Thu, Mar 30, 2023 at 06:34:50AM +0000, Joy Chakraborty wrote:
> > > Check capabilities of DMA controller during init to make sure it is
> > > capable of handling MEM2DEV for tx channel, DEV2MEM for rx channel
> > > and store addr_width capabilities to check per transfer to make sure the
> > > bits/word requirement can be met for that transfer.
> > 
> > ...
> > 
> > > +static enum dma_slave_buswidth dw_spi_dma_convert_width(u8 n_bytes);
> > 
> > Can we avoid forward declarations please?
> > 
> > ...
> > 
> > > +	if (!(tx.directions & BIT(DMA_MEM_TO_DEV) &&
> > > +	      rx.directions & BIT(DMA_DEV_TO_MEM)))
> > > +		return -ENXIO;
> > 
> 
> > What about simplex transfers where we only care about sending or receiving data
> > and using dummy data for the other channel? Doesn't this make a regression for
> > that types of transfers? (Or, if we don't support such, this should be explained
> > in the commit message at least.)
> 
> I don't think the code above is that much relevant for the half-duplex
> transfers. The DW APB SSI-DMA driver requires both Tx and Rx channels
> being specified thus supporting the Full-duplex transfers at least in
> case of the TxRx and Rx-only SPI-transfers (the later case relies on
> having the dummy buffers supplied by the SPI-core). Thus the channels
> must support the corresponding DMA-directions.
> 
> Indeed the Tx-only DMA-based SPI-transfers implementation in the
> driver implies not using the Rx DMA-channel, but even in that case the
> Rx-channel still needs to be specified otherwise the DW APB SSI-DMA
> setup methods will halt with error returned. So unless there are cases
> with dummy Rx DMA-channels (which I very much doubt there is) I don't
> see the suggested update causing a regression. Am I missing something?
> 
> > 
> > ...
> > 
> > > +	/*
> > > +	 * Assuming both channels belong to the same DMA controller hence the
> > > +	 * address width capabilities most likely would be the same.
> > > +	 */
> > > +	dws->dma_addr_widths = tx.dst_addr_widths & rx.src_addr_widths;
> > 
> 
> > I don't think so this is correct.
> > 
> > Theoretically it's possible to have simplex transfers on which the one of
> > the channel is simply ignored / not used. See above.
> 
> Please see my explanation above. To cut it short even in case of the
> half-duplex SPI-xfers both channels need to be specified with the
> respective capabilities. It's implied by the DW APB SSI-DMA setup
> methods design (see dw_spi_dma_init_mfld() and
> dw_spi_dma_init_generic()).
> 
> So until the DW APB SSI-DMA driver is re-developed to supporting true

> Tx-only and Rx-only transfers with no requirement one of the channels

s/with no requirement one of the channels/with no requirement of both the channels

-Serge(y)

> being specified I don't see any problem with the code above. Do you
> still think otherwise?
> 
> -Serge(y)
> 
> > 
> > -- 
> > With Best Regards,
> > Andy Shevchenko
> > 
> >