On Fri, Aug 24, 2018 at 7:45 AM, Sai Prakash Ranjan
[off-list ref] wrote:

read/write{b,w,l,q} are typically used for reading from memory
mapped registers, which can cause hangs if accessed
unclocked. Tracing these events can help in debugging
various issues faced during initial development.

We log this trace information in persistent ram buffer which
can be viewed after reset.

We use pstore_rtb_call() to write the RTB log to pstore.
RTB buffer size is taken from ramoops dt node with additional
property called rtb-size.

For reading the trace after mounting pstore, rtb-ramoops entry
can be seen in /sys/fs/pstore/ as in below sample output.

Sample output of tracing register reads/writes in drivers:

 # mount -t pstore pstore /sys/fs/pstore
 # tail /sys/fs/pstore/rtb-ramoops-0
 [LOGK_READ ] ts:36468476204  data:ffff00000800d0fc  <ffff0000084e9ee0>  gic_check_gicv2+0x58/0x60
 [LOGK_WRITE] ts:36468477715  data:ffff00000800d000  <ffff0000084e9fac>  gic_cpu_if_up+0xc4/0x110
 [LOGK_READ ] ts:36468478548  data:ffff00000800d000  <ffff0000084e9fd8>  gic_cpu_if_up+0xf0/0x110
 [LOGK_WRITE] ts:36468480319  data:ffff00000800d000  <ffff0000084e9fac>  gic_cpu_if_up+0xc4/0x110
 [LOGK_READ ] ts:36468481048  data:ffff00000800d00c  <ffff000008081a34>  gic_handle_irq+0xac/0x128
 [LOGK_WRITE] ts:36468482923  data:ffff00000800d010  <ffff000008081aac>  gic_handle_irq+0x124/0x128
 [LOGK_READ ] ts:36468483184  data:ffff00000800d00c  <ffff000008081a34>  gic_handle_irq+0xac/0x128
 [LOGK_WRITE] ts:36468485215  data:ffff00000800d010  <ffff000008081aac>  gic_handle_irq+0x124/0x128
 [LOGK_READ ] ts:36468486309  data:ffff00000800d00c  <ffff000008081a34>  gic_handle_irq+0xac/0x128
 [LOGK_WRITE] ts:36468488236  data:ffff00000800d010  <ffff000008081aac>  gic_handle_irq+0x124/0x128

Output has below 5 fields:

 * Log type, Timestamp, Data from caller which is the address of
   read/write{b,w,l,q}, Caller ip and Caller name.

Signed-off-by: Sai Prakash Ranjan <redacted>

As this is a tracing-like method, could this instead be added to
ftrace? That would mean it could reuse all the ftrace tools and you'd
get pstore storage for free.

-Kees

-- 
Kees Cook
Pixel Security