Re: [PATCH] USB: Fix incorrect DMA allocations for local memory pool drivers

From: Fredrik Noring <noring@nocrew.org>
To: Christoph Hellwig <hch@lst.de>
Cc: linux-usb@vger.kernel.org,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH] USB: Fix incorrect DMA allocations for local memory pool drivers
Date: Tue, 3 Dec 2019 17:54:07 +0100	[thread overview]
Message-ID: <20191203165407.GA4230@sx9> (raw)
In-Reply-To: <20191203073920.GB23881@lst.de>

Hi Christoph,

> > One can also consider unsetting HCD_DMA for local memory pool drivers.
> 
> That seems like a good idea to me, as the "local DMA pool" really isn't
> DMA in the traditional sense.  The host has to copy data into it by MMIO,
> and it then is accessed by the device.

Perhaps we can combine several enhancements, given that the local memory
pool drivers frequently break with peculiar errors? For example:

1. Arrange localmem_pool and hcd_uses_dma() statements consistently.
   Inconsistent ordering was a source of this bug.

2. Make localmem_pool and hcd_uses_dma() mutually exclusive. Allocating
   DMA memory was a source of this bug.

3. Introduce hcd_uses_localmem_pool(), as show below, to have it on the
   same abstraction level as hcd_uses_dma(). The current localmem_pool
   pointer tests throughout the code are not quite as readable.

A minor note: The commit sequence

7b81cb6bddd2 ("usb: add a HCD_DMA flag instead of guestimating DMA capabilities")
5d6ff300f011 ("usb/max3421: remove the dummy {un,}map_urb_for_dma methods")
bd5defaee872 ("dma-mapping: remove is_device_dma_capable")
cdfee5623290 ("driver core: initialize a default DMA mask for platform device")

broke bisection because the first commit 7b81cb6bddd2 crashes with "DMA map
on device without dma_mask", that is fixed in the fourth commit cdfee5623290.
Too late to do anything about that now, though.

Fredrik

diff --git a/include/linux/usb/hcd.h b/include/linux/usb/hcd.h
--- a/include/linux/usb/hcd.h
+++ b/include/linux/usb/hcd.h
@@ -423,6 +423,11 @@ static inline bool hcd_periodic_completion_in_progress(struct usb_hcd *hcd,
 	return hcd->high_prio_bh.completing_ep == ep;
 }
 
+static inline bool hcd_uses_localmem_pool(struct usb_hcd *hcd)
+{
+	return hcd->localmem_pool != NULL;
+}
+
 static inline bool hcd_uses_dma(struct usb_hcd *hcd)
 {
 	return IS_ENABLED(CONFIG_HAS_DMA) && (hcd->driver->flags & HCD_DMA);
