Re: [PATCH RFC 2/2] dmaengine: fsl-edma: Support dynamic scatter/gather chaining

[Frank Li]

On Tue, May 05, 2026 at 03:51:51PM +0200, Benoît Monin wrote:
> On Monday, 4 May 2026 at 18:04:49 CEST, Frank Li wrote:
> > On Thu, Apr 30, 2026 at 11:49:33AM +0200, Benoît Monin wrote:
> > > Implement dynamic linking of scatter/gather transfers to enable
> > > chaining multiple DMA descriptors without stopping the channel.
> > > This avoids waiting for the channel to go idle if there is another
> > > transaction already issued.
> > >
> > > Add fsl_edma_link_sg() to dynamically link the last TCD of a previously
> > > submitted descriptor to the first TCD of a new descriptor by setting
> > > the scatter/gather address and the E_SG flag, and keeping the channel
> > > active by clearing the DREQ bit.
> >
> > Thank for your trying this, which I want to do long time ago.
> >
> > The key problem is
> >
> >         how to guarratee safe when link to last TCD and DMA is working it?
> >         if update last TCD's next pointer before DMA load it, it is good.
> >         but, if update last TCD's next pointer after DMA load it. DMA engine
> >         may stop.
> >
> I followed what is described in the dynamic scatter/gather chapter of the
> i.MX93 reference manual. We update two registers of the last TCD when
> chaining SG transfers, first TCD_DLAST_SGA then TCD_CSR. This gives us
> three possibilities of concurrence between CPU writes and eDMA reads.
>
> * First case, both registers are updated before the eDMA reads the TCD, we
>   get the expected chaining.
> * Second case, both registers are updated too late and the eDMA have already
>   read the TCD, we are back to the current implementation. After processing
>   the last TCD, the eDMA will disable the channel and the call to
>   fsl_edma_xfer_desc() from fsl_edma_tx_chan_handler() will move to the next
>   issued descriptor and re-enable the channel.
> * Final case, only TCD_DLAST_SGA gets picked up by the eDMA. The eDMA will
>   also disable the channel after processing the last TCD, the only difference
>   is that it will update TCD_DADDR by adding the value TCD_DLAST_SGA. Since
>   we are not reusing TCD_DADDR, this has no impact.

Okay, possible work, let me think more detail after you remove RFC.

>
> >         how do you test it? and how much preformance improved?
> I did my tests by doing SPI transfers with the LPSPI controllers, doing DMA
> transactions with different number of buffers and different buffer sizes.
> Without chaining, interruptions on the SPI bus occur between each DMA
> transaction. With chaining, the activity on the SPI bus is continuous as
> long as DMA transactions are issued before the end of the current
> transaction.

Does SPI support issue new transfers without wait for previous transfer
complete, or SPI transfer already support async queue?

Frank

>
> Best regards,
> --
> Benoît Monin, Bootlin
> Embedded Linux and Kernel engineering
> https://bootlin.com
>
>
>