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Re: [PATCH v2 00/42] esp: consolidate PDMA transfer buffers and other fi

From: Mark Cave-Ayland
Subject: Re: [PATCH v2 00/42] esp: consolidate PDMA transfer buffers and other fixes
Date: Thu, 25 Feb 2021 09:54:20 +0000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0

On 23/02/2021 21:32, Philippe Mathieu-Daudé wrote:

Hi Mark,

On 2/9/21 8:29 PM, Mark Cave-Ayland wrote:
This patch series comes from an experimental branch that I've been working on
to try and boot a MacOS toolbox ROM under the QEMU q800 machine. The effort is
far from complete, but it seems worth submitting these patches separately since
they are limited to the ESP device and form a substantial part of the work to

As part of Laurent's recent q800 work so-called PDMA (pseudo-DMA) support was
added to the ESP device. This is whereby the DREQ (DMA request) line is used
to signal to the host CPU that it can transfer data to/from the device over
the SCSI bus.

The existing PDMA tracks 4 separate transfer data sources as indicated by the
ESP pdma_origin variable: PDMA, TI, CMD and ASYNC with an independent variable
pdma_len to store the transfer length. This works well with Linux which uses a
single PDMA request to transfer a number of sectors in a single request.

Unfortunately the MacOS toolbox ROM has other ideas here: it sends data to the
ESP as a mixture of FIFO and PDMA transfers and then uses a mixture of the FIFO
and DMA counters to confirm that the correct number of bytes have been
transferred. For this to work correctly the PDMA buffers and separate pdma_len
transfer counter must be consolidated into the FIFO to allow mixing of both
types of transfer within a single request.

The patchset is split into several sections:

- Patches 1-7 are minor patches which make esp.c checkpatch friendly, QOMify 
   and also fix up some trace events ready for later patches in the series

- Patches 8-13 unify the DMA transfer count. In particular there are 2 synthetic
   variables dma_counter and dma_left within ESPState which do not need to 
   DMA transfer lengths are programmed into the TC (transfer count) register 
which is
   decremented for each byte transferred, generating an interrupt when it 
reaches zero.
   These patches add helper functions to read the TC and STC registers directly 
   remove these synthetic variables so that the DMA transfer length is now 
tracked in
   a single place.

- Now that the TC register represents the authoritative DMA transfer length, 
   14-25 work to eliminate the separate PDMA variables pdma_start, pdma_cur, 
   and separate PDMA buffers PDMA and CMD. The PDMA position variables can be 
   by the existing ESP cmdlen and ti_wptr/ti_rptr, whilst the FIFO (TI) buffer 
is used
   for incoming data with commands being accumulated in cmdbuf as per standard 

I tried to help reviewing up to this point.

The next parts are too specific to me.

Thanks Phil - I understand that a set of 42 patches for a 25 year old disk controller is never going to be the top of most people's review list, and some parts are almost impossible to review unless you have a good understanding of the datasheet.

I'll see if Laurent has any comments over the next few days, but other than that I'd be inclined to send a v3 followed soon by a PR to avoid me having to update these regularly (I already see a slight conflict with Paolo's SCSI error handling changes, for example).



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