linux-kernel - Re: [PATCH 1/2] mwifiex: Use non-posted PCI register writes

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Message-ID: <eb555433-ade1-e89e-30e4-f4c1c24c25e7@gmail.com>
Date:   Wed, 1 Sep 2021 21:07:41 +0200
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Johannes Berg <johannes@...solutions.net>,
        Pali Rohár <pali@...nel.org>,
        Jonas Dreßler <verdre@...d.nl>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Amitkumar Karwar <amitkarwar@...il.com>,
        Ganapathi Bhat <ganapathi017@...il.com>,
        Xinming Hu <huxinming820@...il.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Tsuchiya Yuto <kitakar@...il.com>,
        "open list:TI WILINK WIRELES..." <linux-wireless@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        Maximilian Luz <luzmaximilian@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH 1/2] mwifiex: Use non-posted PCI register writes

On 01.09.2021 19:07, Johannes Berg wrote:
> On Wed, 2021-09-01 at 18:51 +0200, Heiner Kallweit wrote:
>> On 01.09.2021 17:51, Pali Rohár wrote:
>>> On Wednesday 01 September 2021 16:01:54 Jonas Dreßler wrote:
>>>> On 8/30/21 2:49 PM, Andy Shevchenko wrote:
>>>>> On Mon, Aug 30, 2021 at 3:38 PM Jonas Dreßler <verdre@...d.nl> wrote:
>>>>>>
>>>>>> On the 88W8897 card it's very important the TX ring write pointer is
>>>>>> updated correctly to its new value before setting the TX ready
>>>>>> interrupt, otherwise the firmware appears to crash (probably because
>>>>>> it's trying to DMA-read from the wrong place).
>>>>>>
>>
>> This sounds somehow like the typical case where you write DMA descriptors
>> and then ring the doorbell. This normally requires a dma_wmb().
>> Maybe something like that is missing here?
> 
> But it looks like this "TX ring write pointer" is actually the register?
> 
> However, I would agree that doing it in mwifiex_write_reg() is possibly
> too big a hammer - could be done only for reg->tx_wrptr, not all the
> registers?
> 
> Actually, can two writes actually cross on PCI?
> 
> johannes
> 

In case we're talking about the following piece of code both register
writes are IOMEM writes that are ordered. Maybe the writes arrive properly
ordered but some chip-internal delays cause the issue? Then the read-back
would be something like an ordinary udelay()?
Instead of always reading back register writes, is it sufficient to read
an arbitrary register after mwifiex_write_reg(adapter, reg->tx_wrptr ?


		/* Write the TX ring write pointer in to reg->tx_wrptr */
		if (mwifiex_write_reg(adapter, reg->tx_wrptr,
				      card->txbd_wrptr | rx_val)) {
			mwifiex_dbg(adapter, ERROR,
				    "SEND DATA: failed to write reg->tx_wrptr\n");
			ret = -1;
			goto done_unmap;
		}
		if ((mwifiex_pcie_txbd_not_full(card)) &&
		    tx_param->next_pkt_len) {
			/* have more packets and TxBD still can hold more */
			mwifiex_dbg(adapter, DATA,
				    "SEND DATA: delay dnld-rdy interrupt.\n");
			adapter->data_sent = false;
		} else {
			/* Send the TX ready interrupt */
			if (mwifiex_write_reg(adapter, PCIE_CPU_INT_EVENT,
					      CPU_INTR_DNLD_RDY)) {