linux-kernel - Re: [PATCH 2/2] spi: bcm-mspi: Add support for Broadcom MSPI driver.

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Date:	Wed, 13 May 2015 17:19:06 -0700
From:	Scott Branden <sbranden@...adcom.com>
To:	Jonathan Richardson <jonathar@...adcom.com>,
	Mark Brown <broonie@...nel.org>
CC:	Dmitry Torokhov <dtor@...gle.com>,
	Anatol Pomazau <anatol@...gle.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	"Mark Rutland" <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	<linux-kernel@...r.kernel.org>, <linux-spi@...r.kernel.org>,
	bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
	<devicetree@...r.kernel.org>
Subject: Re: [PATCH 2/2] spi: bcm-mspi: Add support for Broadcom MSPI driver.

Hi Jon,

comment below for full-duplex operation.

On 15-05-13 04:49 PM, Jonathan Richardson wrote:
> On 15-05-12 12:17 PM, Mark Brown wrote:
>> On Tue, May 12, 2015 at 10:38:13AM -0700, Jonathan Richardson wrote:
>>
>>
>>> +	/* The rx data will go into RXRAM0/1 + last tx length. */
>>> +	if (slot + 1 >= NUM_SLOTS)
>>> +		mspi->rx_slot = 0;
>>> +	else
>>> +		mspi->rx_slot = slot + 1;
>>
>> How is this going to work for full duplex transfers if we had to fill
>> the FIFO more than once?
>
> See below, it's not.
>
>>
>>> +static int bcm_mspi_transfer_one(struct spi_master *master,
>>> +	struct spi_device *spidev, struct spi_transfer *transfer)
>>> +{
>>> +	int err;
>>> +
>>> +	/* 8 bit transfers only are currently supported. */
>>> +	if (transfer->bits_per_word > 8)
>>> +		return -ENOTSUPP;
>>
>> Tell the core what the device supports and it will check for you.
>>
>>> +
>>> +	err = bcm_mspi_tx_data(master, spidev, transfer);
>>> +	if (err)
>>> +		return err;
>>> +
>>> +	err = bcm_mspi_rx_data(master, spidev, transfer);
>>> +	if (err)
>>> +		return err;
>>> +
>>> +	return 0;
>>> +}
>>
>> I would expect the recieve and transmit operations to be running in
>> parallel, not executed one after another, given the need to keep
>> manually filling and draining the FIFOs.
>
> The driver was only written with NOR flash in mind as the slave. Since
> this is really just half duplex, it works, though it won't with a real
> full duplex slave. m25p80 never calls transfer_one with both tx and rx
> buffers. The rx bytes that we don't care about from a tx request are
> dropped.
>
> We don't have any full duplex slaves so it's a hard to test. I could
> possibly re-write this so that tx/rx is interleaved and that should be
> good for full duplex or close enough that it won't have to be completely
> overhauled should we ever connect a full duplex slave to it. Maybe I
> should do that. This came from a pre-existing driver that had the same
> limitation.
>
>>

The purpose of this mspi interface is to connect to NOR flash.  There 
are other SPI interfaces on the devices used to connect to other SPI 
devies.  We don't have any need to support full duplex slaves on this 
port (NOR have any hardware wired this way - pun realized after typing 
this).
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