How to see boot error messages

I noticed that the Linux kernel has been updated several times since I installed Debian Jessie and decided to try a new version to see if it would help with a few issues I've been having.

I installed a newer kernel from Debian Backports, but during the reboot I noticed an error message flash by- far too quickly to read.

I was sure I had found a way to read such messages again after the computer finished booting, but just couldn't recall it.

After spending a couple of hours on Google trying to rediscover the secret, I found it.

dmesg

I've put it in red and highlighted it so I don't forget it again.

Running dmesg showed me the error message I'd missed, highlighted in red.

firmware: failed to load radeon/mullins

My new kernel couldn't use the non-free driver firmware I had installed- installing firmware from Debian Backports fixed the problem.

Toshiba C50D-B-120 Part 5 - Firmware update

It's been a few months since I bought my new Toshiba laptop, and by and large, it's been working well. One issue I have had is an occasional artefact in text rendering- I assume a problem with the graphics driver.

I noticed that Debian Backports had some updated firmware for video drivers, so I wondered if this might help with my problem, but I just couldn't see the firmware update even though I have backports in my sources list.

To cut a long story short, the instructions page in the link above has the following line to add to sources list:

deb http://http.debian.net/debian jessie-backports main

To get firmware updates, it should of course be:

deb http://http.debian.net/debian jessie-backports main contrib non-free

(Firmware is non-free software.)

I've upgraded firmware as follows:

Removed the following packages: firmware-linux 
Upgraded the following packages: firmware-linux-nonfree (0.43) to 20151207-1~bpo8+1 
Installed the following packages: firmware-amd-graphics (20151207-1~bpo8+1) firmware-misc-nonfree (20151207-1~bpo8+1) 
Upgraded the following packages: firmware-atheros (0.43) to 20151207-1~bpo8+1

The rendering issue doesn't seem to have entirely disappeared, although first impressions are that it is happening less frequently.